In !7092 (merged), 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 (closed)