Help: Suggest CMAKE_POLICY_DEFAULT_CMP0126 in CMP0126 docs

Help/policy/CMP0126.rst

@@ -22,7 +22,10 @@ will *not* set the cache variable if a non-cache variable of the same name
 already exists and :policy:`CMP0077` is set to ``NEW``.
 
 Policy ``CMP0126`` was introduced in CMake version 3.21. Use the
-:command:`cmake_policy` command to set it to ``OLD`` or ``NEW`` explicitly.
+:command:`cmake_policy` command to set it to ``OLD`` or ``NEW`` explicitly
+within a project.  Use the :variable:`CMAKE_POLICY_DEFAULT_CMP0126
+<CMAKE_POLICY_DEFAULT_CMP\<NNNN\>>` variable to set the policy for
+a third-party project in a subdirectory without modifying it.
 Unlike many policies, CMake version |release| does *not* warn when the policy
 is not set and simply uses ``OLD`` behavior.  See documentation of the
 :variable:`CMAKE_POLICY_WARNING_CMP0126 <CMAKE_POLICY_WARNING_CMP<NNNN>>`