rustdoc: Allow whitespace as path separator like double colon Fixes https://github.com/rust-lang/rust/issues/108447. I think it makes sense since it allows more common cases, however it also makes the syntax heavier. Not sure what the rest of the team thinks about it. In any case we'll need to go through FCP. Full explanation for the changes is available [here](https://github.com/rust-lang/rust/pull/108537#issuecomment-1589480564). r? `@notriddle`