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