Small fixup to rapi docstrings

The fast that most classes don't override the @cvars means that direct
references to FillOpcode can fail (they do on my workstation, but not
on buildbot?). Anyway, for safety, it's best to qualify the name.

Additionally a small typo is fixed.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>