os.version vs macos_version

[Karl-Michael Schindler]

Hallo

As suggested in the discussion of my PR (https://github.com/macports/macports-ports/pull/22460), the question os.version vs macos_version should be discussed here.

The docs about the global variables os.version, os.major,  macos_version and macos_version_major (https://guide.macports.org/#reference.variables) make no difference regarding their usage, contrary to current usage. It should be clearly stated that macos_version is not supposed to be used for os version dependences, but should be limited to examples like error texts. It needs to be stated clearly that only os.version and os.major will be accepted in code. The docs about os.version and os.major should also contain the vital link to https://trac.macports.org/wiki/XcodeVersionInfo#macOSVersionMap 

Regards - Michael aka KaMiSchi