Recommend force-merging after stopping indexing but before marking the index read-only #41687
Description
In #28869 we added a note saying that you should only really force-merge a read-only index. This is a bit misleading: you can't force merge an index that is actually read-only, we really mean to do this on indices that are not going to be written to any more (see comments on #28843). We should clarify these docs to distinguish no-more-indexing from has-a-read-only-block.
Replaces #41624.