doc/dev/cephfs-mirroring: edit file 4 of x

doc/dev/cephfs-mirroring: edit file 4 of x

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.

Signed-off-by: Zac Dover <zac.dover@proton.me>
(cherry picked from commit 26dabda592f8fa2e3a388a474860eb3966aaaac8)