docs/manual: update gettext details

The way gettext is handled in Buildroot has significantly changed,
with changes visible to packages. This commit updates the relevant
section of the manual to document how packages should now interact
with the gettext support.

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