Mention dot-prefixed patterns in hidden index docs (#81464) (#83680)

The hidden index docs did not mention that dot-prefixed patterns default
to matching hidden indices. This PR adds a note explaining the behavior
and why it's like that.

Author: gwbrown

docs/reference/api-conventions.asciidoc

@@ -158,6 +158,11 @@ For most APIs, wildcard expressions do not match hidden data streams and indices
 by default. To match hidden data streams and indices using a wildcard
 expression, you must specify the `expand_wildcards` query parameter.
 
+Alternatively, querying an index pattern starting with a dot, such as
+`.watcher_hist*`, will match hidden indices by default. This is intended to
+mirror Unix file-globbing behavior and provide a smoother transition path to
+hidden indices.
+
 You can create hidden data streams by setting `data_stream.hidden` to `true` in
 the stream's matching <<indices-put-template,index template>>. You can hide
 indices using the <<index-hidden,`index.hidden`>> index setting.