document new lxc-create btrfs behavior

doc/lxc-create.sgml.in

@@ -131,10 +131,12 @@ Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 	    'dir' has the same meaning as 'none', but also allows the optional
 	    <replaceable>--dir ROOTFS</replaceable> to be specified, meaning
 	    that the container rootfs should be placed under the specified path,
-	    rather than the default.
-	    The option 'btrfs' need not be specified as it will be used
-	    automatically if the <filename>@LXCPATH@</filename> filesystem is found to
-	    be btrfs.  If backingstore is 'lvm', then an lvm block device will be
+	    rather than the default.  If 'btrfs' is specified, then the
+	    target filesystem must be btrfs, and the container rootfs will be
+	    created as a new subvolume.  This allows snapshotted clones to be
+	    created, but also causes rsync --one-filesystem to treat it as a
+	    separate filesystem.
+	    If backingstore is 'lvm', then an lvm block device will be
 	    used and the following further options are available:
 	    <replaceable>--lvname lvname1</replaceable> will create an LV
 	    named <filename>lvname1</filename> rather than the default, which