Doc: Be explicit that Pathlib resolve was strict before 3.6. (GH-11316)

Doc/library/pathlib.rst

@@ -976,7 +976,7 @@ call fails (for example because the path doesn't exist).
    is raised.
 
    .. versionadded:: 3.6
-      The *strict* argument.
+      The *strict* argument (pre-3.6 behavior is strict).
 
 .. method:: Path.rglob(pattern)