Auto merge of #43602 - QuietMisdreavus:fantastic-shrinking-headings, r=GuillaumeGomez
Auto merge of #43602 - QuietMisdreavus:fantastic-shrinking-headings, r=GuillaumeGomez

rustdoc: shrink headings in non-top-level docblocks

Headings in per-method docs are often bigger than the function names/signatures themselves, so this tones those down to accentuate the methods.

![screenshot of this change on Vec::swap_remove](https://user-images.githubusercontent.com/5217170/28849380-6116a830-76dc-11e7-8ce2-04433d09463a.png)

Fixes #17193

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