doc/dev/cephfs-mirroring: edit file 3 of x

doc/dev/cephfs-mirroring: edit file 3 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 third quarter of the doc/dev/cephfs-mirroring.rst
file. This commit edits down to the line beginning "`Peers` section in
the command output".

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