manual: patches are not applied for SITE_METHOD = local

We had several remarks on the mailing list of users that were surprised
that patches were not applied for packages whose SITE_METHOD is local.
So document this.

Note that for OVERRIDE_SRCDIR itself it is already documented:

  When Buildroot finds that for a given package, an
  <pkg>_OVERRIDE_SRCDIR has been defined, it will no longer attempt to
  download, extract and patch the package. Instead, it will directly use
  the source code available in in the specified directory.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>