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>