docs: fstab(5) grammar / English fixes, and some other updates

docs: fstab(5) grammar / English fixes, and some other updates

I proofread the whole thing.  I fixed everything that I thought could use
improvement.

various grammar and man page style-guide fixes (commas, word order, etc.).

Reworded a couple things to hopefully make it clear to someone that
didn't already know about fstab.  Re-ordered the intro paragraphs
for easier skimming.  And added an example line.

Expanded on a couple things other things.

Tightened up the wording in some other places to get the point across
faster and in less space.

Thanks to Benno Schulenberg <bensberg@justemail.net>
for several improvements.

Signed-off-by: Peter Cordes <peter@cordes.ca>