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/
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15308544/
4c2ba0ce-1bfc-11e6-9add-
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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