docs: add new percentage-based response option to health check filter (#425)

Signed-off-by: Brian Pane <bpane@pinterest.com>

api/filter/http/health_check.proto

@@ -23,9 +23,8 @@ message HealthCheck {
   // that the filter should cache the upstream response.
   google.protobuf.Duration cache_time = 3;
 
-  // [#not-implemented-hide:]
   // If operating in non-pass-through mode, specifies a set of upstream cluster
   // names and the minimum percentage of servers in each of those clusters that
-  //  must be healthy in order for the filter to return a 200.
+  // must be healthy in order for the filter to return a 200.
   map<string, Percent> cluster_min_healthy_percentages = 4;
 }

docs/root/intro/arch_overview/health_checking.rst

@@ -44,6 +44,11 @@ operation:
 
 * **No pass through**: In this mode, the health check request is never passed to the local service.
   Envoy will respond with a 200 or a 503 depending on the current draining state of the server.
+* **No pass through, computed from upstream cluster health**: In this mode, the health checking
+  filter will return a 200 or a 503 depending on whether at least a :ref:`specified percentage
+  <envoy_api_field_filter.http.HealthCheck.cluster_min_healthy_percentages>` of the
+  servers are healthy in one or more upstream clusters. (If the Envoy server is in a draining
+  state, though, it will respond with a 503 regardless of the upstream cluster health.)
 * **Pass through**: In this mode, Envoy will pass every health check request to the local service.
   The service is expected to return a 200 or a 503 depending on its health state.
 * **Pass through with caching**: In this mode, Envoy will pass health check requests to the local