feat(docs): docstring for `mypyc_attr`

[BobTheBuidler]

I find in-editor tooltips to be very helpful when learning new tools, so I added a simple docstring for mypyc_attr with info I wish I knew originally.

Longer term I'd like to expand this docstring with info on the various attrs and what they do, but I don't want to start on that unless this is approved.