apple_xcframework lacks documentation

Perhaps an oversight? Or because it's quite new?

I'm happy to write something if there isn't already something in progress.

Start by locating the apple_xcframework rule and comparing its existing documentation with documentation for other Apple rules. Determine where this rule should be documented and what usage information is missing. Done means the rule has clear documentation in the project's expected documentation location.

Written by the indexing model from the issue text.