Help: Suggest CMAKE_POLICY_DEFAULT_CMP0077 in CMP0077 docs

Issue: #20490

Help/policy/CMP0077.rst

@@ -53,6 +53,8 @@ between the two policies.
 This policy was introduced in CMake version 3.13.  CMake version
 |release| warns when the policy is not set and uses ``OLD`` behavior.
 Use the :command:`cmake_policy` command to set it to ``OLD`` or ``NEW``
-explicitly.
+explicitly within a project.  Use the :variable:`CMAKE_POLICY_DEFAULT_CMP0077
+<CMAKE_POLICY_DEFAULT_CMP\<NNNN\>>` variable to set the policy for
+a third-party project in a subdirectory without modifying it.
 
 .. include:: DEPRECATED.txt