SOLR-13072: Document the "splitFuzz" parameter.

solr/solr-ref-guide/src/collections-api.adoc

@@ -303,6 +303,10 @@ file that contains the list of deleted documents in each partition. This method
 belonging to the partition. This slows down the replication process and consumes more disk space on replica nodes (the multiple hard-linked
 copies don't occupy additional disk space on the leader node, unless hard-linking is not supported).
 
+`splitFuzz`::
+A float value (default is 0.0f, must be smaller than 0.5f) that allows to vary the sub-shard ranges
+by this percentage of total shard range, odd shards being larger and even shards being smaller.
+
 `property._name_=_value_`::
 Set core property _name_ to _value_. See the section <<defining-core-properties.adoc#defining-core-properties,Defining core.properties>> for details on supported properties and values.
 

solr/solr-ref-guide/src/solrcloud-autoscaling-triggers.adoc

@@ -311,6 +311,14 @@ The `link` method is much faster and puts very little load on the shard leader b
 indexes that are initially as large as the parent shard's index, which slows down replication and
 may lead to excessive initial disk space consumption on replicas.
 
+`splitFuzz`::
+A float value (default is 0.0f, must be smaller than 0.5f) that allows to vary the sub-shard ranges
+by this percentage of total shard range, odd shards being larger and even shards being smaller.
+Non-zero values are useful for large indexes with aggressively growing size, as they help to prevent
+avalanches of split shard requests when the total size of the index
+reaches even multiples of the maximum shard size thresholds.
+
+
 Events generated by this trigger contain additional details about the shards
 that exceeded thresholds and the types of violations (upper / lower bounds, bytes / docs metrics).