Correct the notes on bereq. and beresp.backend in Upgrading to 5.1.

doc/sphinx/whats-new/upgrading-5.1.rst

@@ -73,11 +73,13 @@ Changes to VCL
 Type conversions
 ~~~~~~~~~~~~~~~~
 
-* When ``bereq.backend`` and ``beresp.backend`` are set to a director,
-  then they return an actual backend on subsequent reads if the
-  director resolves to a backend immediately, or the director otherwise.
-  When used in string context, they return the name of the director
-  or of the resolved backend.
+* When ``bereq.backend`` is set to a director, then it returns an
+  actual backend on subsequent reads if the director resolves to a
+  backend immediately, or the director otherwise. If ``bereq.backend``
+  was set to a director, then ``beresp.backend`` returns the backend
+  to which it was set for the fetch.  When either of these is used in
+  string context, it returns the name of the director or of the
+  resolved backend.
 
 * DURATION types may be used in boolean contexts, and are evaluated as
   false when the duration is less than or equal to zero, true