Help: Document CMAKE_OSX_* variables as cache entries

The variables

* CMAKE_OSX_ARCHITECTURES
* CMAKE_OSX_DEPLOYMENT_TARGET
* CMAKE_OSX_SYSROOT

should be set as cache entries.  Document this explicitly.

Help/variable/CMAKE_OSX_VARIABLE.txt

@@ -2,6 +2,8 @@ The value of this variable should be set prior to the first
 :command:`project` or :command:`enable_language` command invocation
 because it may influence configuration of the toolchain and flags.
 It is intended to be set locally by the user creating a build tree.
+This variable should be set as a ``CACHE`` entry (or else CMake may
+remove it while initializing a cache entry of the same name).
 
 Despite the ``OSX`` part in the variable name(s) they apply also to
 other SDKs than macOS like iOS, tvOS, or watchOS.