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 129ea078cd)

sbin/mksnap_ffs/mksnap_ffs.8

@@ -56,7 +56,8 @@ Snapshots can be listed using the
 command.
 .Pp
 After a snapshot is taken it appears as a file.
-The file is the size of the file system from which it was taken.
+The size of the snapshot file is equal to the size of the
+file system from which it was taken.
 It can be moved, renamed or deleted (which removes the snapshot).
 .Pp
 The group ownership of the file is set to