Wrap long `descriptions` for `__doc__` within `MetaDict`.

Already, and foreseeably, we'll be writing pretty long descriptions for
each possible key. When a user runs help, there is nothing to stop this
line from going on and on and on and on and on and on and on and on and on and on and on and on and...
forever.

I capped the length to 62, which is 70 CPL - 2*TABSIZE, where TABSIZE=4.

plotly's python-api help, now actually readable :)