aufs: doc abi, compatible to sphinx or reStructuredText

In Documentation/ABI/testing/{debug,sys}fs-aufs, I used a notation as
	brabraN, brabra1 ... brabraN
But it caused an error in "make htmldocs".

Reported-by: GI Jack
See-also: https://github.com/sfjro/aufs5-standalone/issues/4

Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Documentation/ABI/testing/debugfs-aufs

@@ -28,7 +28,7 @@ Description:
 		When the aufs mount option 'noxino' is specified, it
 		will be empty. About XINO files, see the aufs manual.
 
-What:		/debug/aufs/si_<id>/xi0, xi1 ... xiN and xiN-N
+What:		/debug/aufs/si_<id>/xi<branch-index>
 Date:		March 2009
 Contact:	J. R. Okajima <hooanon05g@gmail.com>
 Description:

Documentation/ABI/testing/sysfs-aufs

@@ -6,14 +6,14 @@ Description:
 		per aufs mount, where <id> is a unique id generated
 		internally.
 
-What:		/sys/fs/aufs/si_<id>/br0, br1 ... brN
+What:		/sys/fs/aufs/si_<id>/br<idx>
 Date:		March 2009
 Contact:	J. R. Okajima <hooanon05g@gmail.com>
 Description:
 		It shows the abolute path of a member directory (which
 		is called branch) in aufs, and its permission.
 
-What:		/sys/fs/aufs/si_<id>/brid0, brid1 ... bridN
+What:		/sys/fs/aufs/si_<id>/brid<idx>
 Date:		July 2013
 Contact:	J. R. Okajima <hooanon05g@gmail.com>
 Description: