Become flag-compatible with existing rustdoc

[steveklabnik]

https://github.com/rust-lang/rust/blob/master/src/doc/rustdoc/src/command-line-arguments.md describes the flags that current rustdoc accepts. For each flag, we should:

1. Decide if we want to keep the flag or not
2. For the ones we may not keep, implement them, but issue a warning
3. For the ones we do, implement them!

I'm going to make an isssue per flag, as we should have those discussions individually.