debugfs: Add a reference to the debugfs API documentation.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Cc: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

lib/Kconfig.debug

@@ -74,6 +74,9 @@ config DEBUG_FS
 	  debugging files into.  Enable this option to be able to read and
 	  write to these files.
 
+	  For detailed documentation on the debugfs API, see
+	  Documentation/DocBook/filesystems.
+
 	  If unsure, say N.
 
 config HEADERS_CHECK