Documentation: git-commit -a

A bit more elaboration on what "update all paths" means.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/git-commit.txt

@@ -25,7 +25,9 @@ information.
 OPTIONS
 -------
 -a|--all::
-	Update all paths in the index file.
+	Update all paths in the index file.  This flag notices
+	files that have been modified and deleted, but new files
+	you have not told about git are not affected.
 
 -c or -C <commit>::
 	Take existing commit object, and reuse the log message