doc: remove possibly confusing comment

There are valid reasons to use the other keymap-creation functions, if
one needs them. On the other hand, if one is supposed to use RMLVO, it
is more or less the only choice, so the comment is not needed in this
case as well.

Signed-off-by: Ran Benita <ran234@gmail.com>
doc: extend xkb_rule_names default-value description

Especially a mention of the XKB_DEFAULT_* envvars was missing.

Reported-by: Paeglis Gatis <Gatis.Paeglis@digia.com> (thanks!)
Signed-off-by: Ran Benita <ran234@gmail.com>