doc/cephfs: Cosmetic changes and small fixes in cephfs-mirroring.rst

doc/cephfs: Cosmetic changes and small fixes in cephfs-mirroring.rst

Don't format an unordered list as preformatted block.

Fix (not linked) reference to a section name that doesn't exist.

Don't call systemd unit files "systemctl unit files", add an "and"
between them.

Clarify by means of inline formatting and whitespace that config value
is "60" and the unit for that is seconds (as opposed to value being
"60s").

Use double backticks consistently when listing commands, file names or
paths, command parameters, syntaxes, etc.

Use star for inline italic instead of single backtick (to differentiate
from the previous).

Don't use double backticks for "cephadm" when talking about the concept.

Don't capitalize a few words that didn't need it.
Capitalize table data for consistency.
All caps for "MDS".

Small updates to language in text.

Call it "cephfs-mirror" everywhere and not "cephfs_mirror" at one place.

Signed-off-by: Ville Ojamo <14869000+bluikko@users.noreply.github.com>