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

It took me some digging through the issues to discover that the python import
```python
from x import f as g
```
can be translated in julia to
```julia
using PythonCall
@py import x: f as g
```
Can this be added to the PythonCall's guide in the documentation?

Start with the PythonCall guide in the documentation and find the section covering Python imports. Add the documented Julia syntax for translating “from ... import ...” with an alias, using the example from the issue; the guide should clearly show the equivalent Python and Julia forms.

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