Snapshot files are set with the "snapshot" file flag during creation by
mksnap_ffs(8) or mount(8). The previous statement was incorrect since
it mentions the immutable flag, i.e. "noschg" which is not the case when
one does "ls -lo" in a directory containing snapshot files.

While I am here, mention a way for finding snapshot files on a file
system.

PR: docs/80535 (inspired by)