Adjust sizing guidance re. doc count (elastic#97831)

In elastic#87246 we describe some reasons why it's a good idea to limit the doc
count of a shard, and we started to do so in elastic#94065, so this commit
adjusts the sizing guidance docs to match.

Author: DaveCTurner

docs/reference/how-to/size-your-shards.asciidoc

@@ -140,20 +140,21 @@ Every new backing index is an opportunity to further tune your strategy.
 
 [discrete]
 [[shard-size-recommendation]]
-==== Aim for shard sizes between 10GB and 50GB
-
-Larger shards take longer to recover after a failure. When a node fails, {es}
-rebalances the node's shards across the data tier's remaining nodes. This
-recovery process typically involves copying the shard contents across the
-network, so a 100GB shard will take twice as long to recover than a 50GB shard.
-In contrast, small shards carry proportionally more overhead and are less
-efficient to search. Searching fifty 1GB shards will take substantially more
-resources than searching a single 50GB shard containing the same data.
-
-There are no hard limits on shard size, but experience shows that shards
-between 10GB and 50GB typically work well for logs and time series data. You
-may be able to use larger shards depending on your network and use case.
-Smaller shards may be appropriate for
+==== Aim for shards of up to 200M documents, or with sizes between 10GB and 50GB
+
+There is some overhead associated with each shard, both in terms of cluster
+management and search performance. Searching a thousand 50MB shards will be
+substantially more expensive than searching a single 50GB shard containing the
+same data. However, very large shards can also cause slower searches and will
+take longer to recover after a failure.
+
+There is no hard limit on the physical size of a shard, and each shard can in
+theory contain up to just over two billion documents. However, experience shows
+that shards between 10GB and 50GB typically work well for many use cases, as
+long as the per-shard document count is kept below 200 million.
+
+You may be able to use larger shards depending on your network and use case,
+and smaller shards may be appropriate for
 {enterprise-search-ref}/index.html[Enterprise Search] and similar use cases.
 
 If you use {ilm-init}, set the <<ilm-rollover,rollover action>>'s