docs/manual: update documentation about <pkg>_SOURCE, <pkg>_PATCH and <pkg>_EXTRA_DOWNLOADS

This commit updates the Buildroot manual for the variables used to
indicate where to download the source code from:

 - It updates the description of <pkg>_SOURCE to make it clear that
   Buildroot assume that the tarball is hosted at <pkg>_SITE.

 - It updates the description of <pkg>_PATCH to indicate that not only
   file names (assumed to be hosted at <pkg>_SITE) can be used, but
   also full URLs. This allows to match with what the current code is
   doing.

 - It updates the description of <pkg>_EXTRA_DOWNLOADS to also
   indicate that full URLs are now accepted, following the change made
   in the previous commit.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Romain Naour <romain.naour@openwide.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>