Docs: remove experimental tag from update-by-query
We meant to remove it in but we removed it from the
documentation for the java client but not the rest documetnaiton.
[[docs-update-by-query]]
== Update By Query API
-experimental[The update-by-query API is new and should still be considered experimental. The API may change in ways that are not backwards compatible]
-
The simplest usage of `_update_by_query` just performs an update on every
document in the index without changing the source. This is useful to
<<picking-up-a-new-property,pick up a new property>> or some other online
