Docs: Explain _bulk?refresh shard targeting

Only the shards that receive the bulk request will be affected by
`refresh`. Imagine a `_bulk?refresh=wait_for` request with three
documents in it that happen to be routed to different shards in an index
with five shards. The request will only wait for those three shards to
refresh. The other two shards of that make up the index do not
participate in the `_bulk` request at all.

Relates to #31819

docs/reference/docs/bulk.asciidoc

@@ -230,6 +230,13 @@ example.
 Control when the changes made by this request are visible to search. See
 <<docs-refresh,refresh>>.
 
+NOTE: Only the shards that receive the bulk request will be affected by
+`refresh`. Imagine a `_bulk?refresh=wait_for` request with three
+documents in it that happen to be routed to different shards in an index
+with five shards. The request will only wait for those three shards to
+refresh. The other two shards of that make up the index do not
+participate in the `_bulk` request at all.
+
 [float]
 [[bulk-update]]
 === Update