[DOCS] Added compound_on_flush docs and updated compound_format

docs to include note about accepting a float

docs/reference/index-modules.asciidoc

@@ -15,14 +15,26 @@ all the relevant modules settings can be provided when creating an index
 There are specific index level settings that are not associated with any
 specific module. These include:
 
-`index.compound_format`::
+[[index-compound-format]]`index.compound_format`::
 
-	Should the compound file format be used (boolean
-	setting). If not set, controlled by the actually store used, this is
-	because the compound format was created to reduce the number of open
-	file handles when using file based storage. By default, it is set to
-	`false` for better performance (really applicable for file system based
-	index storage), but, requires adapting the max open file handles.
+	Should the compound file format be used (boolean setting).
+	The compound format was created to reduce the number of open
+	file handles when using file based storage. However, by default it is set
+	to `false` as the non-compound format gives better performance. It is important
+	that OS is configured to give Elasticsearch ``enough'' file handles.
+    See <<file-descriptors>>.
++
+Alternatively, `compound_format` can be set to a number between `0` and
+`1`, where `0` means `false`, `1` means `true` and a number inbetween
+represents a percentage: if the merged segment is less than this
+percentage of the total index, then it is written in compound format,
+otherwise it is written in non-compound format.  added[0.90.2]
+
+[[index-compound-on-flush]]`index.compound_on_flush`::
+
+    Should a new segment (create by indexing, not by merging) be written
+    in compound format or non-compound format? Defaults to `true`.
+    This is a dynamic setting.
 
 `index.term_index_interval`::
 

docs/reference/index-modules/merge.asciidoc

@@ -36,44 +36,53 @@ over-merge (i.e., cascade merges).
 
 This policy has the following settings:
 
-[cols="<,<",options="header",]
-|=======================================================================
-|Setting |Description
-|`index.merge.policy.expunge_deletes_allowed` |When expungeDeletes is
-called, we only merge away a segment if its delete percentage is over
-this threshold. Default is `10`.
+`index.merge.policy.expunge_deletes_allowed`::
 
-|`index.merge.policy.floor_segment` |Segments smaller than this are
-"rounded up" to this size, i.e. treated as equal (floor) size for merge
-selection. This is to prevent frequent flushing of tiny segments from
-allowing a long tail in the index. Default is `2mb`.
+	When expungeDeletes is called, we only merge away a segment if its delete
+	percentage is over this threshold. Default is `10`.
 
-|`index.merge.policy.max_merge_at_once` |Maximum number of segments to
-be merged at a time during "normal" merging. Default is `10`.
+`index.merge.policy.floor_segment`::
 
-|`index.merge.policy.max_merge_at_once_explicit` |Maximum number of
-segments to be merged at a time, during optimize or expungeDeletes.
-Default is `30`.
+	Segments smaller than this are "rounded up" to this size, i.e. treated as
+	equal (floor) size for merge selection. This is to prevent frequent
+	flushing of tiny segments from allowing a long tail in the index. Default
+	is `2mb`.
 
-|`index.merge.policy.max_merged_segment` |Maximum sized segment to
-produce during normal merging (not explicit optimize). This setting is
-approximate: the estimate of the merged segment size is made by summing
-sizes of to-be-merged segments (compensating for percent deleted docs).
-Default is `5gb`.
+`index.merge.policy.max_merge_at_once`::
 
-|`index.merge.policy.segments_per_tier` |Sets the allowed number of
-segments per tier. Smaller values mean more merging but fewer segments.
-Default is `10`. Note, this value needs to be >= then the
-`max_merge_at_once_` otherwise you'll force too many merges to occur.
+	Maximum number of segments to be merged at a time during "normal" merging.
+	Default is `10`.
 
-|`index.reclaim_deletes_weight` |Controls how aggressively merges that
-reclaim more deletions are favored. Higher values favor selecting merges
-that reclaim deletions. A value of `0.0` means deletions don't impact
-merge selection. Defaults to `2.0`.
+`index.merge.policy.max_merge_at_once_explicit`::
 
-|`index.compound_format` |Should the index be stored in compound format
-or not. Defaults to `false`.
-|=======================================================================
+	Maximum number of segments to be merged at a time, during optimize or
+	expungeDeletes. Default is `30`.
+
+`index.merge.policy.max_merged_segment`::
+
+	Maximum sized segment to produce during normal merging (not explicit
+	optimize). This setting is approximate: the estimate of the merged segment
+	size is made by summing sizes of to-be-merged segments (compensating for
+	percent deleted docs). Default is `5gb`.
+
+`index.merge.policy.segments_per_tier`::
+
+	Sets the allowed number of segments per tier. Smaller values mean more
+	merging but fewer segments. Default is `10`. Note, this value needs to be
+	>= then the `max_merge_at_once_` otherwise you'll force too many merges to
+	occur.
+
+`index.reclaim_deletes_weight`::
+
+	Controls how aggressively merges that reclaim more deletions are favored.
+	Higher values favor selecting merges that reclaim deletions. A value of
+	`0.0` means deletions don't impact merge selection. Defaults to `2.0`.
+
+`index.compound_format`::
+
+	Should the index be stored in compound format or not. Defaults to `false`.
+	See <<index-compound-format,`index.compound_format`>> in
+	<<index-modules-settings>>.
 
 For normal merging, this policy first computes a "budget" of how many
 segments are allowed by be in the index. If the index is over-budget,

docs/reference/indices/update-settings.asciidoc

@@ -31,8 +31,6 @@ curl -XPUT 'localhost:9200/my_index/_settings' -d '
 Below is the list of settings that can be changed using the update
 settings API:
 
-[horizontal]
-
 `index.number_of_replicas`::
     The number of replicas each shard has.
 
@@ -140,9 +138,14 @@ settings API:
     Either `simple` or `buffered` (default).
 
 `index.compound_format`::
-    See <<index-modules>>
+    See <<index-compound-format,`index.compound_format`>> in
+    <<index-modules-settings>>.
 
-|<<index-modules-slowlog>>::
+`index.compound_on_flush`::
+    See <<index-compound-on-flush,`index.compound_on_flush>> in
+    <<index-modules-settings>>.
+
+<<index-modules-slowlog>>::
     All the settings for slow log.
 
 `index.warmer.enabled`::