Add a brief description of CTLFLAG_VNET flag. Since the VIMAGE is
absolutely not documented, I see no reason in long descriptions
here.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271615 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Add comments describing r271604 change.

MFC after:	3 days


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271616 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f