Rollup merge of #78678 - Nemo157:doc-cfg-w-traits, r=jyn514,GuillaumeGomez

Rollup merge of #78678 - Nemo157:doc-cfg-w-traits, r=jyn514,GuillaumeGomez

Add tests and improve rendering of cfgs on traits

Shows the additional features required to get the trait implementation, suppressing any already shown on the current page. One interesting effect from this is if you have a cfg-ed type, implementing a cfg-ed trait (so the implementation depends on both cfgs), you will get the inverted pair of cfgs shown on each page:

![image](https://user-images.githubusercontent.com/81079/97904671-207bdc00-1d41-11eb-8144-707e8017d2b6.png)

![image](https://user-images.githubusercontent.com/81079/97904700-27a2ea00-1d41-11eb-8b9f-e925ba339044.png)

The hidden items on the trait implementation also now get the correct cfgs displayed on them.

Tests are blocked on #78673.

fixes #68100
cc #43781