Config.in.legacy: update description for developers

The existing comments in Config.in.legacy are not entirely in-line with
current practice. The comments implies that BR2_LEGACY should not be set when
the conversion from old-to-new symbol can be done automatically using the
appropriate 'select' statements. However, none of the existing legacy options
does it this way. Moreover, I think it's intentional that the user is notified
of the change, so that the removal of the legacy options in later buildroot
versions no longer poses a problem.
Additionally, the comments now describe how to handle string options.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>