Document the 72 char limit on package description.

Document the 72 char limit on package description.

While describing a package I though that the tab and 2 spaces was
already doing the margin for wrapping the help test (ie 70 char when
wrapping to 80) but apparently the full text need to fit 72 columns.

So this patch propose to document that fact in the two places where
the Config.in format is described.

Signed-off-by: Julien Viard de Galbert <julien@vdg.name>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>