Minor documentation changes

Also, replace [apply symmetry] with [symmetry], because that works now.

Using [(*] doesn't show up in coqdoc.  Using [(**] does.
Merge pull request #175 from JasonGross/minor-improvements

Minor documentation changes.

I see you started to use coqdoc documentation.
Would there be a way to generate this automatically using Travis??
Merge branch 'master' of https://github.com/HoTT/HoTT