Docs fixups

[dougo]

Purpose (TL;DR) - mandatory

Fix some broken formatting and links in the docs.

How to verify - mandatory

1. Check out this branch (see github instructions below)
2. cd docs
3. bundle
4. jekyll serve
5. View API docs at http://localhost:4000/releases/v1.17.7/stubs/ and other pages.

[coveralls]

Coverage remained the same at 95.379% when pulling c9367aa on dougo:docs-fixups into 86088bd on sinonjs:master.

[mantoni]

Fantstic, thank you!

[cjohansen]

Thanks!