Help: Document Use_wxWindows.cmake as deprecated

The implementation already logged a message saying that the module
was deprecated. This change adds that message to the module docs.

Help/manual/cmake-modules.7.rst

@@ -268,6 +268,7 @@ Deprecated Utility Modules
    /module/CMakeExpandImportedTargets
    /module/CMakeForceCompiler
    /module/TestCXXAcceptsFlag
+   /module/Use_wxWindows
    /module/WriteBasicConfigVersionFile
 
 Deprecated Find Modules

Modules/Use_wxWindows.cmake

@@ -5,7 +5,8 @@
 # Use_wxWindows
 # -------------
 #
-#
+# Deprecated.  Use ``find_package(wxWidgets)`` and
+# ``include(${wxWidgets_USE_FILE})`` instead.
 #
 #
 # This convenience include finds if wxWindows is installed and set the