legal-info: add a comment about what packages we save the tarballs of

As the legal-info infra only (rightfully) saves the tarballs of packages
that:
  - we want to redistribute,
  - and are not local,
  - and are not overriden,

add a comment stating so.

This should clarify the code-block, which although trivial to read,
was not easy to interpret without thinking thouroughly about it.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Fabio Porcedda <fabio.porcedda@gmail.com>
Reviewed-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/pkg-generic.mk

@@ -627,6 +627,9 @@ ifneq ($$($(2)_LICENSE_FILES),)
 $(1)-legal-info: $(1)-extract
 endif
 
+# We only save the sources of packages we want to redistribute, that are
+# non-local, and non-overriden. So only store, in the manifest, the tarball
+# name of those packages.
 ifeq ($$($(2)_REDISTRIBUTE),YES)
 ifneq ($$($(2)_SITE_METHOD),local)
 ifneq ($$($(2)_SITE_METHOD),override)