rustdoc: add stability and source links to the methods; w/ style tweaks

- Pull-in changes from #13802, with a rebase
- Fix again link selectors (unrelated to the original PR)
- Show stability attributes with color pellets, add more explicit titles that will appear on hover
- Keep using `[src]` for the source links, don't show them on methods until (method+description) is hovered

Example: http://adrientetar.legtux.org/cached/rust-docs/foo.htm

cc @lifthrasiir, closes #13802.

r? @alexcrichton