doc/cephfs: edit disaster-recovery-experts (3 of x)

doc/cephfs: edit disaster-recovery-experts (3 of x)

Edit the fifth and sixth 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.

The sections edited in this commit are

 - MDS Map Reset
 - Recovery From Mission Metadata Objects

Follows https://github.com/ceph/ceph/pull/61427

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

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