unshare: add more hints about mount namespaces to the man page

There is also idea (by Lennart) to add --propagation=[shared|private|slave|off]
to unshare(1), but it seems unnecessary and too complex as everyone can call
mount(8) after unshare(1) to setup a proper namespace.

Signed-off-by: Karel Zak <kzak@redhat.com>