MAINTAINERS: describe differences between F: and N: patterns

There is a difference in how scripts/get_maintainer.pl treats
F: and N: file pattern matches.

Describe those differences in the MAINTAINERS file.

Signed-off-by: Joe Perches <joe@perches.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Mark Brown <broonie@linaro.org>
Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>