gh-85417: Clarify behaviour on branch cuts in cmath module (#102046)

This PR updates the cmath module documentation to reflect the reality that Python is almost always (and as far as I can tell, that "almost" can be omitted) running on a machine whose C double supports signed zeros.

* Removes misleading references to functions being continuous from above / below / the left / the right at branch cuts
* Expands the note on branch cuts at the top of the module documentation to explain the double-sided sign-of-zero-based behaviour