Outdated docs on privileges to use RUM sourcemapping

[carsonip]

Docs on privileges to perform RUM sourcemapping is outdated. Since the introduction of ES sourcemap fetcher in #9722 , to use ES sourcemap fetcher, it requires read on .apm-source-map.

Also, the right place for the docs should be here.

Ideally backport the doc update to 8.7+ since the required privilege is new from 8.7.