FFS/UFS snapshots: improve documentation.

This update provides a more detailed description of FFS/UFS snapshots
and adds links to useful man pages.

Requested by: Graham Perrin
Submitted by: darius-dons.net.au
Reviewed by: Ravi Pokala
PR: 266358
Differential Revision: https://reviews.freebsd.org/D38817
Approved by: re (gjb)

(cherry picked from commit 528d8e55c8d338e78fa4e6c9f182e8975f75199a)