Help/cmake_minimum_required: clarify policy_max `versionadded` note

While the prose clarifies this below, a quick glance at the docs can
make one believe that `...3.10` has no sensible use. However, it is
still useful to tell CMake 3.12 and up that at least 3.10 behavior is
accepted.

Help/command/cmake_minimum_required.rst

@@ -8,7 +8,7 @@ Require a minimum version of cmake.
   cmake_minimum_required(VERSION <min>[...<policy_max>] [FATAL_ERROR])
 
 .. versionadded:: 3.12
-  The optional ``<policy_max>`` version.
+  The optional ``<policy_max>`` version behavior; ignored in older CMake.
 
 Sets the minimum required version of cmake for a project.
 Also updates the policy settings as explained below.