docs/manual: update documentation about gettext handling

This commit updates the Buildroot manual regarding how gettext related
issues are handled, now that we have cases where the gettext package
should be selected even with glibc toolchains.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>