improvement: Explicitly do not support tuple-upackings as it is unclear how to assign the associated docstring and value.

[NiklasRosenstein]

@nrser This is in response to your comment in the code. While this section isn't particularly pretty (is any of the docspec code, really? 💦), I think it unreasonable to try and handle the tuple-unpacking scenario in any way because any semantics we assign will be inaccurate in the current way Docspec represents variable assignments.

This change should clear up what names can be though.