doc/cephfs: edit 3rd 3rd of mount-using-kernel-driver

doc/cephfs: edit 3rd 3rd of mount-using-kernel-driver

Edit the third third of doc/cephfs/mount-using-kernel-driver.rst in
preparation for correcting mount commands that may not work in Reef as
described in this documentation.

This commit edits only English-language strings in
doc/cephfs/mount-using-kernel-driver.rst. No technical content (that is,
no commands and no settings) have been altered in this commit.

Technical alterations to this file will be made only after the English
is unambiguous.

This PR follows the following two PRs:
https://github.com/ceph/ceph/pull/61048 - 1st 3rd
https://github.com/ceph/ceph/pull/61049 - 2nd 3rd

Co-authored-by: Anthony D'Atri <anthony.datri@gmail.com>
Signed-off-by: Zac Dover <zac.dover@proton.me>