doc/cephfs: edit disaster-recovery-experts (4 of x)

doc/cephfs: edit disaster-recovery-experts (4 of x)

Edit the seventh and final section of
doc/cephfs/disaster-recovery-experts.rst in preparation for adding
deeper explanations of the contexts in which one should use the various
commands listed on that page.

The section edited in this commit is

 * Using an alternate metadata pool for recovery

A future commit might beneficially put this section into the format of
an ordered list. If so, such a commit should only reformat the
content and should not make any changes to the English. It's enough to
verify content or format. Let's not overload our editorial faculties by
forcing ourselves to walk and chew gum at the same time.

Follows https://github.com/ceph/ceph/pull/61442

https://tracker.ceph.com/issues/69557

Signed-off-by: Zac Dover <zac.dover@proton.me>