Rollup merge of #61505 - ebarnard:doc-shrink, r=GuillaumeGomez

Only show methods that appear in `impl` blocks in the Implementors sections of trait doc pages

In the "Implementors" and "Implementations on Foreign Types" sections, only show methods that appear in the `impl` block for that type. This has the benefit of
- Reducing the size of the Iterator page, and other large trait documentation pages.
- Retaining documentation on the `impl` blocks and functions in the `impl` blocks.
- Indicating which provided methods are overridden.
- Making the documentation match the structure of the code being documented.
- Being a small change that can be easily backed out if issues arise.

A set of Rust stdlib docs build with this change are [available here](https://ebarnard.github.io/2019-06-03-rust-smaller-trait-implementers-docs/).

The size of the [`Iterator` doc page](https://ebarnard.github.io/2019-06-03-rust-smaller-trait-implementers-docs/std/iter/trait.Iterator.html) is reduced from 14.4MB (latest nightly) to 724kB.

Before:
<img width="1411" alt="Screenshot 2019-06-03 at 23 12 17" src="https://user-images.githubusercontent.com/1059683/58837971-1722a780-8655-11e9-8d81-51e48130951d.png">

After:
<img width="1428" alt="Screenshot 2019-06-03 at 16 41 27" src="https://user-images.githubusercontent.com/1059683/58814907-84ffac80-861e-11e9-8692-79be473a5299.png">

cc #55900

0 files changed, 0 insertions, 0 deletions