doc/style.css: make selectors more selective.

doc/style.css: make selectors more selective.

Change #foo to div#foo to avoid selecting headings
with anchor foo, such as <h1 id="foo">.

(A more extensive change would be to use class
selectors for styling. Perhaps this is better, since id:s
should be unique within a document according to
http://www.w3.org/TR/CSS2/selector.html#id-selectors)

R=golang-dev, gri, adg
CC=golang-dev
https://golang.org/cl/5702044