[DOCS] Re-add paragraph noting doc_count is approximate (#83154) (#83159)

This paragraph was accidentally removed as part of #79205. Also fixes a minor heading capitalization error.

(cherry picked from commit 63f228e)

Author: jrodewig

docs/reference/aggregations/bucket/terms-aggregation.asciidoc

@@ -129,7 +129,7 @@ aggregation close to the `max_buckets` limit. You'll know you've gone too large
 if the request fails with a message about `max_buckets`.
 
 [[search-aggregations-bucket-terms-aggregation-shard-size]]
-==== Shard Size
+==== Shard size
 
 To get more accurate results, the `terms` agg fetches more than
 the top `size` terms from each shard. It fetches the top `shard_size` terms,
@@ -157,6 +157,10 @@ NOTE:   `shard_size` cannot be smaller than `size` (as it doesn't make much sens
 [[terms-agg-doc-count-error]]
 ==== Document count error
 
+Even with a larger `shard_size` value, `doc_count` values for a `terms`
+aggregation may be approximate. As a result, any sub-aggregations on the `terms`
+aggregation may also be approximate.
+
 `sum_other_doc_count` is the number of documents that didn't make it into the
 the top `size` terms. If this is greater than `0`, you can be sure that the
 `terms` agg had to throw away some buckets, either because they didn't fit into