Documentation: Add missing links to `DEBUG_POSTFIX` target property

Add links to the `<CONFIG>_POSTFIX` target property when it is
referenced in the `DEBUG_POSTFIX` documentation.

Help/prop_tgt/DEBUG_POSTFIX.rst

@@ -1,7 +1,7 @@
 DEBUG_POSTFIX
 -------------
 
-See target property ``<CONFIG>_POSTFIX``.
+See target property :prop_tgt:`<CONFIG>_POSTFIX`.
 
-This property is a special case of the more-general ``<CONFIG>_POSTFIX``
+This property is a special case of the more-general :prop_tgt:`<CONFIG>_POSTFIX`
 property for the ``DEBUG`` configuration.