syntax: Correct span for provided trait methods.
rustdoc: A quick CSS tweak for stability and source links.

this makes a source link much more visible (and link-like), and
makes a stability index more consistent for the top-level headings
and the method signature. this should be better than nothing,
but further adjustments and/or redesigns are welcomed.

Conflicts:
	src/librustdoc/html/static/main.css