Problem:
- Lots of redundant text in options docs for "not allowed in
a modeline", even though we already have a flag that indicates that.
- `deny_in_modelines` is an old vestigial flag only used by 'encoding'
(which never changes).
Solution:
- Generate docs based on the `secure` flag.
- Remove the `deny_in_modelines` flag (`kOptFlagNoML`).