manual: update for multiple global patch dirs

Updating the documentation to reflect that multiple directories can
now be specified for BR2_GLOBAL_PATCH_DIR. Along with giving an
example use case of how to use multiple global patch directories.

[Thomas: fix minor typo mentionned by Thomas De Schampheleire.]

Signed-off-by: Ryan Barnett <rjbarnet@rockwellcollins.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>