Help: Clarify standard policy advice paragraph wording on how policies are set

Avoid implying that a policy must always be set explicitly.

Help/policy/STANDARD_ADVICE.txt

@@ -1,3 +1,3 @@
 This policy was introduced in CMake version |INTRODUCED_IN_CMAKE_VERSION|.
-Use the :command:`cmake_policy` command to set it to ``OLD`` or ``NEW`` explicitly.
+It may be set by :command:`cmake_policy` or :command:`cmake_minimum_required`.
 If it is not set, CMake |WARNS_OR_DOES_NOT_WARN|, and uses ``OLD`` behavior.