doc/cephfs: edit disaster-recovery-experts (2 of x)

doc/cephfs: edit disaster-recovery-experts (2 of x)

Edit the third and fourth sections 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.

Follows https://github.com/ceph/ceph/pull/61426

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

Co-authored-by: Anthony D'Atri <anthony.datri@gmail.com>
Signed-off-by: Zac Dover <zac.dover@proton.me>