Rollup merge of #80362 - jyn514:rustc-macros, r=ehuss
Rollup merge of #80362 - jyn514:rustc-macros, r=ehuss

Document rustc_macros on nightly-rustc

Fixes https://github.com/rust-lang/rust/issues/80345.

![image](https://user-images.githubusercontent.com/23638587/103113442-b7ba2d00-4628-11eb-8a4d-c542f2d170e1.png)
![image](https://user-images.githubusercontent.com/23638587/103113448-bc7ee100-4628-11eb-8657-2d72e88de656.png)

r? ``@ehuss``

graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust