Documentation of preludes is handled inconsistently

## How can Bevy's documentation be improved?

> - bevy_time has nothing on the prelude module, and doc hidden on the reexports. this seems to be the most common case
> - bevy_transform has doc hidden on the prelude and on the reexports. there are a handful of crates that way

- @mockersf

Our goals here:
- multiple copies of the same item should not appear in the docs: only the canonical path should appear
- users should be able to see what's in the preludes