Q#'s ArcTan2 function

[kuzminrobin]

If needed, document in more details the Q#'s ArcTan2 function, in particular whether it corresponds to the C (C++) semantics' atan2().
More details.

Extend QSharpFoundation unit tests to cover what has been noticed about ArcTan2, so that we can prevent regressions that would cause QIR and C# runtimes to disagree (more info).

[cgranade]

Transferring to the runtime repo, as that repo contains the API documentation comments for ArcTan2 and similar functions.