doc/cephfs: edit 2nd 3rd of mount-using-kernel-driver

doc/cephfs: edit 2nd 3rd of mount-using-kernel-driver

Edit the second 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.

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