Config.in.legacy: document how to handle renaming choice options

When a choice option is renamed, it is not possible to select the new
option. Instead, the default for the new option should be set in the
choice itself. So let's document this.

While we're at it, also reformat the whitespace in the example so a tab
is used at the appropriate place.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Martin Bark <martin@barkynet.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>