coccinelle.txt: update documentation to include M= option

Adding documentation for the new M= option which limits Coccinelle
to a specific set of directories.
Signed-off-by: Greg Dietsche <Gregory.Dietsche@cuw.edu>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Michal Marek <mmarek@suse.cz>

Documentation/coccinelle.txt

@@ -102,9 +102,15 @@ or
 	make coccicheck COCCI=<my_SP.cocci> MODE=report
 
 
- Using Coccinelle on (modified) files
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ Controlling Which Files are Processed by Coccinelle
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+By default the entire kernel source tree is checked.
+
+To apply Coccinelle to a specific directory, M= can be used.
+For example, to check drivers/net/wireless/ one may write:
 
+    make coccicheck M=drivers/net/wireless/
+    
 To apply Coccinelle on a file basis, instead of a directory basis, the
 following command may be used: