Additionally:
* Included the existing update by query java api docs in java-api docs.
(for some reason it was never included, it needed some tweaking and
then it was good to go)
* moved delete-by-query / update-by-query code samples to java file so
that we can verify that these samples at least compile.
Closes#24203
The reindex API is mature now, and we will work to maintain backwards
compatibility in accordance with our backwards compatibility
policy. This commit unmarks the reindex API as experimental.
Relates #23621
