Document status quo regarding req.backend_hint and restarts

Related to #2305

doc/sphinx/users-guide/vcl-actions.rst

@@ -31,7 +31,8 @@ The most common actions to return are these:
 
 *restart*
   Restart processing of the request. You can restart the processing of
-  the whole transaction. Changes to the `req` object are retained.
+  the whole transaction. Changes to the `req` object are retained, except
+  for `req.backend_hint`, which gets reset to the default backend.
 
 *retry*
   Retry the request against the backend. This can be returned from

lib/libvcc/generate.py

@@ -330,6 +330,8 @@ sp_variables = [
 		or the director otherwise.
 		When used in string context, returns the name of the director
 		or backend, respectively.
+		Note: backend_hint gets reset to the default backend by
+		restarts!
 		"""
 	),
 	('req.hash_ignore_busy',