docs/manual: clarify the <PKG>_PATCH_DEPENDENCIES guarantee

Unlike <PKG>_DEPENDENCIES, <PKG>_PATCH_DEPENDENCIES only guarantees
extract and patch of listed dependencies, not build. Make this subtlety
more explicit in the documentation.

Cc: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
[yann.morin.1998@free.fr: slight fix]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit d01e808bfed8f76e084ec2899bd130a47fb020fa)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/adding-packages-generic.txt

@@ -374,9 +374,9 @@ not and can not work as people would expect it should:
 * +LIBFOO_PATCH_DEPENDENCIES+ lists the dependencies (in terms of
   package name) that are required for the current package to be
   patched. These dependencies are guaranteed to be extracted and
-  patched before the current package is patched. In a similar way,
-  +HOST_LIBFOO_PATCH_DEPENDENCIES+ lists the dependencies for the
-  current host package.
+  patched (but not necessarily built) before the current package is
+  patched. In a similar way, +HOST_LIBFOO_PATCH_DEPENDENCIES+ lists
+  the dependencies for the current host package.
   This is seldom used; usually, +LIBFOO_DEPENDENCIES+ is what you
   really want to use.