[docs][PythonCall] document syntax for "from ... import ..."

[CarloLucibello]

It took me some digging through the issues to discover that the python import

from x import f as g

can be translated in julia to

using PythonCall
@py import x: f as g 

Can this be added to the PythonCall's guide in the documentation?

[cjdoris]

Sure, please make a PR to update the docstring for @py.