ht: Complain if TNone isn't listed first for TOr/TAnd

Some type descriptions are rather long. If "None" is listed at the end
or somewhere in between it is easily missed. Therefore it should be at
the beginning, e.g. "None or (long description)".

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>