Help: XCFrameworks and stubs are supported on all Apple platforms, not just macOS

The support for XCFrameworks and import libraries/stubs was developed separately and over a couple of CMake feature releases. There is a degree of overlap between these two features, but the documentation was updated more or less independently. This MR merges and unifies some of the relevant statements related to these features so that they better reflect an overall understanding.

It also seems that the existing docs focus on macOS in cases where a feature actually applies to all Apple platforms. Specifically, the implementation of import libraries/stubs doesn't seem to differentiate between the different Apple platforms, and the feature should indeed apply to all of them in most cases. The one exception is that on non-macOS, a non-framework import library is of no practical use since you can't deploy a bare shared library to iOS, tvOS, etc. and use it at run time.

This MR seeks to address the above points. I added some missing versionadded directives while I was at it, since they became more relevant with these changes.

Backport: release