Updated Doc string in Base.Generator

[r0cketr1kky]

Solves #35226

[JeffBezanson]

First, I don't think if guards should be mentioned here, since if you have one you don't get a Generator but a Filter iterator. (I realize this PR didn't introduce that, but we might as well fix it while we're at it.)

The original wording was also chosen for a reason. The point is that a Generator happens to be what you get when you write x for x in y. It's not the only syntax for constructing a Generator; Generator(f, itr) also works like for any type.