Feedback from @pnkfelix's review:

- explicitly disallowing attributes on `if`/`else` due to weirdness
- provide alternatives for this
- add a drawbacks section discussing syntactic lock-down and the
  antipattern of `#[cfg]`
- clarify that this RFC is just for parsing attributes, not requiring that
  `#[cfg]` stripping be added in all these new places.