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

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

Edit the first 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 is a cherry-pick from a branch that targeted the Reef
release branch. After some thought I realized that there was no reason
that the Engliish grammar shouldn't be clean in this branch too.

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