The public APIs are documented on developer.apple.com in the iOS Reference Library.
However, the only absolutely authentic source on whether their use is acceptable is to submit an app and have it reviewed. Apple just added a review board if you with to appeal a review ruling, so that may be the new last word (unless you get the executive staff's attention (e.g. SJ)).
If you wish more facts before submitting an app, there are a few sites which show which types of apps are being accepted and rejected, and if so, for what given reason. However past acceptance of a type of app is not a precedent or guarantee for any future policy.
If you wish to try interpreting their rules and guidelines yourself, they are available as part of the Developer iOS Standard Agreement.
The Developer support people who answer technical question usually cannot answer review or approval questions, except to point you at the proper API documentation. (The reason may be that these are often legal, corporate policy or marketing questions, not technical questions.)