Mark the search 'fields' option as beta. (#63699)

We've identified two important enhancements that may affect the API. We expect
any API changes from these enhancements to be minor, but want to leave open the
possibility for small breaks. For example, we may end up returning unmapped
fields by default, or omitting nested fields from the root hit. The impact to
users should be quite small.

We're tracking the issues we need to resolve before removing the 'beta' label
here: #60985.

docs/reference/search/search-your-data/retrieve-selected-fields.asciidoc

@@ -60,6 +60,7 @@ following sections:
 [discrete]
 [[search-fields-param]]
 === Fields
+beta::[]
 
 The `fields` parameter allows for retrieving a list of document fields in
 the search response. It consults both the document `_source` and the index