Add prompts (and perform necessary corrections to glaring grammatical
errors) to doc/dev/cephfs-mirroring.rst, as requested by Jos Collin in
https://github.com/ceph/ceph/pull/63237/files#r2085886075.
This commit edits the fourth (and final) quarter of the
doc/dev/cephfs-mirroring.rst file.
Further refinements to the English in this file are possible.