[online-3.7] Router - changes to A/B weights discussion

Bug: 1477685
https://bugzilla.redhat.com/show_bug.cgi?id=1477685

Trello: BhWCH3vu
https://trello.com/c/BhWCH3vu/543-3-fix-problem-in-a-b-weight-algorithm
(cherry picked from commit 591a32b) xref:openshift#5816

Author: pecameron

architecture/topics/alternate_backends_weights.adoc

@@ -3,22 +3,23 @@ A route is usually associated with one service through the `to:` token with
 the service based on the xref:load-balancing[load balancing strategy].
 
 It is possible to have as many as four services supporting the route. The
-portion of  requests that are handled by each service is governed by the service
+portion of requests that are handled by each service is governed by the service
 `weight`.
 
 The first service is entered using the `to:` token as before, and up to three
 additional services can be entered using the `alternateBackend:` token. Each
 service must be `kind: Service` which is the default.
 
-Each service can have a `weight` associated with it. The portion of requests
+Each service has a `weight` associated with it. The portion of requests
 handled  by the service is `weight` / `sum_of_all_weights`. When a service has
 more than one  endpoint, the service's weight is distributed among the endpoints
 with each endpoint getting at least 1. If the service `weight` is 0 each
-endpoint will get 0.
+of the service's endpoints will get 0.
 
 The `weight` must be in the range 0-256. The default is 1. When the `weight` is
 0 no requests are passed to the service. If all services have `weight` 0,
-requests are returned with a 503 error. 
+requests are returned with a 503 error.  When a sevice has no endpoints, the weight
+is effectively 0.
 
 When using `alternateBackends` also use the `roundrobin`
 xref:load-balancing[load balancing strategy] to ensure requests are distributed
@@ -59,4 +60,5 @@ spec:
 <2> The first service name is `service-name` which may have 0 or more pods
 <3> The alternateBackend services may also have 0 or more pods
 <4> The total `weight` is 40. `service-name` will get 20/40 or 1/2 of the requests,
-`service-name2` and `service-name3` will each get 1/4 of the requests.
+`service-name2` and `service-name3` will each get 1/4 of the requests, assuming each
+service has 1 or more endpoints.