Merge topic 'doc-apple-arch-default' into release-4.2

6055a12d36 Help: Document default architecture when CMAKE_OSX_ARCHITECTURES is not set

Acked-by: Kitware Robot <[email protected]>
Acked-by: rumgot rumgotov <[email protected]>
Merge-request: !11296

Help/variable/CMAKE_OSX_ARCHITECTURES.rst

@@ -7,4 +7,8 @@ This variable is used to initialize the :prop_tgt:`OSX_ARCHITECTURES`
 property on each target as it is created.  See that target property
 for additional information.
 
+If ``CMAKE_OSX_ARCHITECTURES`` is not set, the compiler's default target
+architecture is used.  For compilers provided by Xcode, this is the host
+machine's architecture.
+
 .. include:: include/CMAKE_OSX_VARIABLE.rst