Document most everything.

Some things are left undocumented, where they are uninteresting or REALLY
self-apparent. Adds `#[deny(missing_doc)]` to `render` and `device`, but also
some well-placed `#[allow(missing_doc)]`.

Closes #114
"if the stencil test passes" - this is incorrect.
There are 3 operations (see "op_*" members), two for failures.