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>