Documentation: explain the difference between __bitwise and __bitwise__

Simply added explanation from Al Viro in the following mail:
http://lkml.indiana.edu/hypermail/linux/kernel/0802.2/3164.html

Cc: Al Viro <viro@ftp.linux.org.uk>
Cc:  Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/Documentation/sparse.txt b/Documentation/sparse.txt
index 42f43fa..34c76a5 100644
--- a/Documentation/sparse.txt
+++ b/Documentation/sparse.txt
@@ -42,6 +42,14 @@
 vs cpu-endian vs whatever), and there the constant "0" really _is_
 special.
 
+__bitwise__ - to be used for relatively compact stuff (gfp_t, etc.) that
+is mostly warning-free and is supposed to stay that way.  Warnings will
+be generated without __CHECK_ENDIAN__.
+
+__bitwise - noisy stuff; in particular, __le*/__be* are that.  We really
+don't want to drown in noise unless we'd explicitly asked for it.
+
+
 Getting sparse
 ~~~~~~~~~~~~~~