Docs for libstd's prelude.rs should mention what is reexported

Right now the only way to know what src/libstd/prelude.rs reexports is to visit the source directly. This should be mentioned in the docs.

In addition reexported modules could be highlighted on the modules list page for convenience.

[thestinger]

Closing in favour of #5486 which is a more general solution. In incoming, there aren't actually re-exported modules anymore but the pub use statements that are there should really be listed.