Help/CMP0102: mention the varible that controls CMP0102 warnings

This was missed in the review of !4150, but was noticed during
discussion of #22353.

Help/policy/CMP0102.rst

@@ -22,6 +22,8 @@ already exist in the cache.
 This policy was introduced in CMake version 3.17.  Use the
 :command:`cmake_policy` command to set it to ``OLD`` or ``NEW`` explicitly.
 Unlike many policies, CMake version |release| does *not* warn
-when this policy is not set and simply uses ``OLD`` behavior.
+when this policy is not set and simply uses ``OLD`` behavior.  See
+documentation of the :variable:`CMAKE_POLICY_WARNING_CMP0102
+<CMAKE_POLICY_WARNING_CMP<NNNN>>` variable to control the warning.
 
 .. include:: DEPRECATED.txt