man: virsh: Document lxc-enter-namespace --noseclabel

https://bugzilla.redhat.com/show_bug.cgi?id=1147737
(cherry picked from commit 0f8be253)

tools/virsh.pod

@@ -4170,7 +4170,7 @@ When I<--timestamp> is used, a human-readable timestamp will be printed
 before the event, and the timing information provided by QEMU will be
 omitted.
 
-=item B<lxc-enter-namespace> I<domain> -- /path/to/binary [arg1, [arg2, ...]]
+=item B<lxc-enter-namespace> I<domain> [I<--noseclabel>] -- /path/to/binary [arg1, [arg2, ...]]
 
 Enter the namespace of I<domain> and execute the command C</path/to/binary>
 passing the requested args. The binary path is relative to the container
@@ -4179,6 +4179,10 @@ environment variables / console visible to virsh. This command only works
 when connected to the LXC hypervisor driver.  This command succeeds only
 if C</path/to/binary> has 0 exit status.
 
+By default the new process will run with the security label of the new
+parent container. Use the I<--noseclabel> option to instead have the
+process keep the same security label as C<virsh>.
+
 =back
 
 =head1 ENVIRONMENT