Use "caller" instead of "global" to describe where imports go

[apjanke]

This passage talks about import copying names into the global namespace. But doesn't it copy names into the calling module's namespace instead?

[mpoulin]

@apjanke I don't like the word caller because you don't call a module in the sense of calling a function or class method. What about saying "the local namespace" instead? That matches the terminology used in the official Python docs.

[apjanke]

Yes, that makes sense.

I've updated this PR to do that.