Add some IsTrunc typeclass documentation

Also, reorder Overture.v a bit so we can put all the truncation code in
one place.  Perhaps the typeclass machinery should go with tactics?
Maybe?
Merge pull request #165 from JasonGross/document-typeclasses

Add some IsTrunc typeclass documentation
Merge branch 'master' of https://github.com/HoTT/HoTT