[DOCS] Reformat split index API docs (#46713) (#47578)

docs/reference/indices/split-index.asciidoc

@@ -4,6 +4,58 @@
 <titleabbrev>Split index</titleabbrev>
 ++++
 
+Splits an existing index into a new index with more primary shards.
+
+[source,console]
+----
+POST /twitter/_split/split-twitter-index
+{
+  "settings": {
+    "index.number_of_shards": 2
+  }
+}
+----
+// TEST[s/^/PUT twitter\n{"settings":{"blocks.write":true}}\n/]
+
+
+[[split-index-api-request]]
+==== {api-request-title}
+
+`POST /<index>/_shrink/<target-index>`
+
+`PUT /<index>/_shrink/<target-index>`
+
+
+[[split-index-api-prereqs]]
+==== {api-prereq-title}
+
+
+Before you can split an index:
+
+* The index must be read-only.
+* The <<cluster-health, cluster health>> status must be green.
+
+You can do make an index read-only
+with the following request:
+
+[source,console]
+--------------------------------------------------
+PUT /my_source_index/_settings
+{
+  "settings": {
+    "index.blocks.write": true <1>
+  }
+}
+--------------------------------------------------
+// TEST[s/^/PUT my_source_index\n/]
+
+<1> Prevents write operations to this index while still allowing metadata
+    changes like deleting the index.
+
+
+[[split-index-api-desc]]
+==== {api-description-title}
+
 The split index API allows you to split an existing index into a new index,
 where each original primary shard is split into two or more primary shards in
 the new index.
@@ -34,27 +86,28 @@ index may by split into an arbitrary number of shards greater than 1.  The
 properties of the default number of routing shards will then apply to the
 newly split index.
 
-[float]
-==== How does splitting work?
 
-Splitting works as follows:
+[[how-split-works]]
+===== How splitting works
 
-* First, it creates a new target index with the same definition as the source
+A split operation:
+
+. Creates a new target index with the same definition as the source
   index, but with a larger number of primary shards.
 
-* Then it hard-links segments from the source index into the target index. (If
+. Hard-links segments from the source index into the target index. (If
   the file system doesn't support hard-linking, then all segments are copied
   into the new index, which is a much more time consuming process.)
 
-* Once the low level files are created all documents will be `hashed` again to delete
+. Hashes all documents again, after low level files are created, to delete
   documents that belong to a different shard.
 
-* Finally, it recovers the target index as though it were a closed index which
+. Recovers the target index as though it were a closed index which
   had just been re-opened.
 
-[float]
+
 [[incremental-resharding]]
-==== Why doesn't Elasticsearch support incremental resharding?
+===== Why doesn't Elasticsearch support incremental resharding?
 
 Going from `N` shards to `N+1` shards, aka. incremental resharding, is indeed a
 feature that is supported by many key-value stores. Adding a new shard and
@@ -83,49 +136,16 @@ covers both the old and the new index for read operations. Assuming that the
 old and new indices have respectively +M+ and +N+ shards, this has no overhead
 compared to searching an index that would have +M+N+ shards.
 
-[float]
-==== Preparing an index for splitting
 
-Create a new index:
-
-[source,console]
---------------------------------------------------
-PUT my_source_index
-{
-  "settings": {
-    "index.number_of_shards" : 1
-  }
-}
---------------------------------------------------
-
-In order to split an index, the index must be marked as read-only,
-and have <<cluster-health,health>> `green`.
-
-This can be achieved with the following request:
-
-[source,console]
---------------------------------------------------
-PUT /my_source_index/_settings
-{
-  "settings": {
-    "index.blocks.write": true <1>
-  }
-}
---------------------------------------------------
-// TEST[continued]
-
-<1> Prevents write operations to this index while still allowing metadata
-    changes like deleting the index.
-
-[float]
-==== Splitting an index
+[[split-index]]
+===== Split an index
 
 To split `my_source_index` into a new index called `my_target_index`, issue
 the following request:
 
 [source,console]
 --------------------------------------------------
-POST my_source_index/_split/my_target_index
+POST /my_source_index/_split/my_target_index
 {
   "settings": {
     "index.number_of_shards": 2
@@ -159,7 +179,7 @@ and accepts `settings` and `aliases` parameters for the target index:
 
 [source,console]
 --------------------------------------------------
-POST my_source_index/_split/my_target_index
+POST /my_source_index/_split/my_target_index
 {
   "settings": {
     "index.number_of_shards": 5 <1>
@@ -177,8 +197,9 @@ POST my_source_index/_split/my_target_index
 
 NOTE: Mappings may not be specified in the `_split` request.
 
-[float]
-==== Monitoring the split process
+
+[[monitor-split]]
+==== Monitor the split process
 
 The split process can be monitored with the <<cat-recovery,`_cat recovery`
 API>>, or the <<cluster-health, `cluster health` API>> can be used to wait
@@ -196,9 +217,36 @@ split process begins. When the split operation completes, the shard will
 become `active`. At that  point, Elasticsearch will try to allocate any
 replicas and may decide to relocate the primary shard to another node.
 
-[float]
-==== Wait For Active Shards
+
+[[split-wait-active-shards]]
+==== Wait for active shards
 
 Because the split operation creates a new index to split the shards to,
 the <<create-index-wait-for-active-shards,wait for active shards>> setting
 on index creation applies to the split index action as well.
+
+
+[[split-index-api-path-params]]
+==== {api-path-parms-title}
+
+`<index>`::
+(Required, string)
+Name of the source index to split.
+
+include::{docdir}/rest-api/common-parms.asciidoc[tag=target-index]
+
+
+[[split-index-api-query-params]]
+==== {api-query-parms-title}
+
+include::{docdir}/rest-api/common-parms.asciidoc[tag=wait_for_active_shards]
+
+include::{docdir}/rest-api/common-parms.asciidoc[tag=timeoutparms]
+
+
+[[split-index-api-request-body]]
+==== {api-request-body-title}
+
+include::{docdir}/rest-api/common-parms.asciidoc[tag=target-index-aliases]
+
+include::{docdir}/rest-api/common-parms.asciidoc[tag=target-index-settings]