In 2a8df7e7 (productbuild: Don't write rootVolumeOnly attribute if
writing domains, 2022-03-21), the variable holding the main contents
of the distribution.dist XML file was renamed from
CPACK_PACKAGEMAKER_CHOICES to CPACK_APPLE_PKG_INSTALLER_CONTENT.
This reflected the fact that the PackageMaker generator is deprecated.
The new variable also includes more details than the old one held.
Some projects were relying on the old variable name, so we need to still
set that to the same contents as it would have previously provided.

Neither of these variables were previously documented, but the older
variable was mentioned in enough semi-official places that it essentially
became semi-supported. Document both variables and highlight that the
older one is deprecated.

Fixes: #23467