FFS/UFS snapshots: improve documentation for removal (deletion) and unlinking.
Clarification of the size of the snapshot file.
Suggested by: Matteo Riondato
PR: 266358
Differential Revision: https://reviews.freebsd.org/D38817
Approved by: re (gjb)
(cherry picked from commit 129ea078cd16b768a1100b497282fd9359e8dfea)