Hide private definitions in rustdoc

[brson]

Rustdoc used to be able to do this, but forgot. Now there are a number of #[doc(hidden)] attributes floating around to make up for the deficiency.

[z0w0]

I'm surprised this wasn't fixed earlier, it's as simple as uncommenting a line of code.

[thestinger]

Fixed by 387aa36.

[brson]

@z0w0 that's amusing