Addons: deprecate 'wiki_url'

When running with debug enabled ('-d' argument),
warnings are printed for add-ons which are not yet updated.

Reminder to name things based on what they do,
not the technologies they use :)

doc/python_api/examples/bpy.types.AddonPreferences.1.py

@@ -6,7 +6,7 @@ bl_info = {
     "location": "SpaceBar Search -> Add-on Preferences Example",
     "description": "Example Add-on",
     "warning": "",
-    "wiki_url": "",
+    "doc_url": "",
     "tracker_url": "",
     "category": "Object",
 }