Rollup merge of #33679 - Manishearth:rustdoc-readmore-impls, r=alexcrichton

Rollup merge of #33679 - Manishearth:rustdoc-readmore-impls, r=alexcrichton

rustdoc: Add doc snippets for trait impls, with a read more link

The read more link only appears if the documentation is more than one line long.

![screenshot from 2016-05-17 06 54 14](https://cloud.githubusercontent.com/assets/1617736/15308544/4c2ba0ce-1bfc-11e6-9add-29de8dc7ac6e.png)

It currently does not appear on non-defaulted methods, since you can document them directly. I could make it so that default documentation gets forwarded if regular docs don't exist.

Fixes #33672

r? @alexcrichton

cc @steveklabnik