glabel.8: Warn against using generic labels on a shared device

Also suggest against creating a generic label on a device which already
contains a filesystem.

PR: 264166
Reviewed by: imp, delphij, Pau Amma <pauamma@gundo.com>
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D35326

(cherry picked from commit 81f36fbc98dd74ca923938e0329919d426811b0c)