As of version 10.12 (Sierra), the name of Apple's desktop operating
system will be macOS. Change all occurrences where the Mac platform
is discussed to use the macro \macos (defined in the documentation
configuration in qtbase).

Also, fix the usage instructions for related Q_OS_* macros.

Change-Id: I5bac6f787a4c434d7ff83036a2cfea4eef41cdc1
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Jake Petroules <jake.petroules@qt.io>