Allow force-merge on read-only indices

[DaveCTurner]

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). Yet maybe we can permit force-merging on a read-only index? If not we should clarify these docs.

[elasticmachine]

Pinging @elastic/es-distributed

[DaveCTurner]

We discussed this today and noted that it might indeed be possible to do this, but the notion of "read-only" is not very well-defined. We contemplated a number of ideas and concluded that fixing the docs is the right way forwards.

[DaveCTurner]

Superseded by #41687.