Improve docs for split API in 6.1/6.x (#27504)

Author: clintongormley

docs/reference/indices/split-index.asciidoc

@@ -1,23 +1,25 @@
 [[indices-split-index]]
 == Split Index
 
-number_of_routing_shards
-
-The split index API allows you to split an existing index into a new index
-with multiple of it's primary shards. Similarly to the <<indices-shrink-index,Shrink API>>
-where the number of primary shards in the shrunk index must be a factor of the source index.
-The `_split` API requires the source index to be created with a specific number of routing shards
-in order to be split in the future. (Note: this requirement might be remove in future releases)
-The number of routing shards specify the hashing space that is used internally to distribute documents
-across shards, in oder to have a consistent hashing that is compatible with the method elasticsearch
-uses today.
-For example an index with `8` primary shards and a `index.number_of_routing_shards` of `32`
-can be split into `16` and `32` primary shards. An index with `1` primary shard
-and `index.number_of_routing_shards` of `64` can be split into `2`, `4`, `8`, `16`, `32` or `64`.
-The same works for non power of two routing shards ie. an index with `1` primary shard and
-`index.number_of_routing_shards` set to `15` can be split into `3` and `15` or alternatively`5` and `15`.
-The number of shards in the split index must always be a factor of `index.number_of_routing_shards`
-in the source index. Before splitting, a (primary) copy of every shard in the index must be active in the cluster.
+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.
+
+IMPORTANT:  The `_split` API requires the source index to be created with a
+specific `number_of_routing_shards` in order to be split in the future. This
+requirement has been removed in Elasticsearch 7.0.
+
+The number of times the index can be split (and the number of shards that each
+original shard can be split into) is determined by the
+`index.number_of_routing_shards` setting. The number of routing shards
+specifies the hashing space that is used internally to distribute documents
+across shards with consistent hashing. For instance, a 5 shard index with
+`number_of_routing_shards` set to `30` (`5 x 2 x 3`) could be split by a
+factor of `2` or `3`.  In other words, it could be split as follows:
+
+* `5` -> `10` -> `30`  (split by 2, then by 3)
+* `5` -> `15` -> `30` (split by 3, then by 2)
+* `5` -> `30` (split by 6)
 
 Splitting works as follows:
 
@@ -29,7 +31,7 @@ Splitting works as follows:
   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
-  documents that belong in a different shard.
+  documents that belong to a different shard.
 
 * Finally, it recovers the target index as though it were a closed index which
   had just been re-opened.