umount.2: Tweak's to Eric Biederman's "shared subtrees on umount" patch

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/umount.2

@@ -97,23 +97,38 @@ Don't dereference
 if it is a symbolic link.
 This flag allows security problems to be avoided in set-user-ID-\fIroot\fP
 programs that allow unprivileged users to unmount filesystems.
-
-.P
-Shared mount points cause any mount activity on that mount point
-including umounts to be forwarded to every shared mount point in it's
-peer group and every slave mount of that peer group.  This means that
-umount of any peer in a set of shared mounts will cause all of it's
+.PP
+Shared mount points cause any mount activity on a mount point, including
+.BR umount (2)
+operations, to be forwarded to every shared mount point in the
+peer group and every slave mount of that peer group.
+This means that
+.BR umount (2)
+of any peer in a set of shared mounts will cause all of its
 peers to be unmounted and all of their slaves to be unmounted as well.
 
-This propogagtion of unmount activity can be particularly surprising
-on systems where every mount point is shared by default.  On such
-systems recursively bind mounting the root directory of the filesystem
+This propagation of unmount activity can be particularly surprising
+on systems where every mount point is shared by default.
+On such systems,
+recursively bind mounting the root directory of the filesystem
 onto a subdirectory and then later unmounting that subdirectory with
 .BR MNT_DETACH
 will cause every mount in the mount namespace to be lazily unmounted.
 
-To ensure umount does not propagate the mount point may be
-remounted with MS_REC | MS_PRIVATE prior to umount being called.
+To ensure
+.BR umount (2)
+does not propagate in this fashion,
+the mount point may be remounted using a
+.BR mount (2)
+call with a
+.I mount_flags
+argument that includes both
+.BR MS_REC
+and
+.BR MS_PRIVATE
+prior to
+.BR umount (2)
+being called.
 
 .SH RETURN VALUE
 On success, zero is returned.