Help: Document CMP0118 requirement for boolean values

Help/policy/CMP0118.rst

@@ -10,10 +10,16 @@ property of the source.  Consequently, the associated ``GENERATED``
 property is now visible from any directory scope, not only from the scope
 for which it was set.
 
+Additionally, the ``GENERATED`` property may now be set only to boolean
+values, and may not be turned off once turned on.
+
 The ``OLD`` behavior of this policy is to only allow ``GENERATED`` to be
 visible from the directory scope for which it was set.  The ``NEW``
 behavior on the other hand allows it to be visible from any scope.
 
-This policy was introduced in CMake version 3.20.  CMake version |release|
-warns when the policy is not set and uses ``OLD`` behavior. Use the
+This policy was introduced in CMake version 3.20.  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 with regard
+to visibility of the ``GENERATED`` property.  However, CMake does warn
+about setting the ``GENERATED`` property to a non-boolean value.