Rustdoc should render the `Fn` traits with the sugary syntax

[ftxqxd]

Rustdoc currently displays Fn, FnMut, and FnOnce like Fn<(Args,), Return>. That syntax is unsightly and will not be usable post-1.0, so it should probably render them as Fn(Args) -> Return instead.

[tomjakubowski]

this is a dupe of #19877 but since that one never got tagged I'll close it. I've got a patch almost ready for this.