Rollup merge of #34513 - ollie27:rustdoc_stripped, r=alexcrichton

Rollup merge of #34513 - ollie27:rustdoc_stripped, r=alexcrichton

rustdoc: Fix a few stripping issues

We need to recurse into stripped modules to strip things like impl methods
but when doing so we must not add any items to the `retained` set.

For example this removes [`core::num::ParseFloatError::__description`](https://doc.rust-lang.org/nightly/core/num/struct.ParseFloatError.html#method.__description) and [`impl Clone for ThreadRng`](https://doc.rust-lang.org/nightly/std/clone/trait.Clone.html).