Update zfs-snapshot.8

Fixes a small inaccuracy in the description of snapshot
atomicity

zfs-snapshot(8) appears to contain a small error. The existing
version reads "Snapshots are taken atomically, so that all
snapshots correspond to the same moment in time." Per
zfs_main.c, which in do_snapshot() simply loops over argv, this
does not appear to be correct when multiple snapshots are
specified explicitly on the command line. I believe the intent
of the man page was to say that *recursive* snapshots are all
created atomically.

This proposed change fixes that error. Because the existing
statement may confuse some readers anyway, the commit also also
adds a small amount of general explanatory information that may
be helpful.

The change also adds an introductory sentence that summarizes
what 'zfs snapshot' does in the first place. In that sentence,
the text "different datasets" is intended to indicate that
(again per the code) the same dataset cannot be specified
multiple times on the command line.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Shawn Bayern <sbayern@law.fsu.edu>
Closes #15857