Documentation: mention scripts/diffconfig tool

The kconfig documentation suggests using plain 'diff' to compare config
files and then adds "Yes, we need something better here".  Commit
a717417e7f96 ("kconfig: add diffconfig utility") added what that comment
was looking for.

Signed-off-by: Javi Merino <javi.merino@arm.com>
Cc: Michal Marek <mmarek@suse.cz>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/kbuild/kconfig.txt

@@ -28,12 +28,10 @@ new (default) values, so you can use:
 
 	grep "(NEW)" conf.new
 
-to see the new config symbols or you can 'diff' the previous and
-new .config files to see the differences:
+to see the new config symbols or you can use diffconfig to see the
+differences between the previous and new .config files:
 
-	diff .config.old .config | less
-
-(Yes, we need something better here.)
+	scripts/diffconfig .config.old .config | less
 
 ______________________________________________________________________
 Environment variables for '*config'