shard director: explain how the ring is constructed with weights

 Conflicts:
	lib/libvmod_directors/vmod.vcc

lib/libvmod_directors/vmod.vcc

@@ -307,7 +307,9 @@ structure gets built from the last 32 bits of SHA256 hash values of
 `<ident>`\ `<n>` (default `ident` being the backend name) for each
 backend and for a running number `n` from 1 to `replicas`. Hashing
 creates the seemingly random order for placement of backends on the
-consistent hashing ring.
+consistent hashing ring.  When ``.add_backend()`` is called with a
+weight argument, replicas is scaled by that weight to add
+proportionally more copies of the that backend on the ring.
 
 When ``.backend()`` is called, a load balancing key gets generated
 unless provided. The smallest hash value in the circle is looked up