Auto merge of #43445 - zackmdavis:rustdoc_major_section_self-links, r=GuillaumeGomez
Auto merge of #43445 - zackmdavis:rustdoc_major_section_self-links, r=GuillaumeGomez

rustdoc: make major section headers self-links

The sidebar already has links to these h2's ids, but for convenience,
the h2 itself should also be a link (retaining its present appearance).

![doc_sec_self-links](https://user-images.githubusercontent.com/1076988/28503806-cdce4e00-6fc1-11e7-832a-2ed1fc62808f.png)

This should address most of #24484.

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