docs: clarify hc host default (#3385)

The default is actually the cluster name, not the ip

Signed-off-by: Snow Pettersen <snowp@squareup.com>

Mirrored from https://github.com/envoyproxy/envoy @ 2fa152da07db067cb0aedd1ef309759cb9424de6

envoy/api/v2/core/health_check.proto

@@ -61,8 +61,8 @@ message HealthCheck {
 
   message HttpHealthCheck {
     // The value of the host header in the HTTP health check request. If
-    // left empty (default value), the IP on behalf of which this health check is performed will be
-    // used.
+    // left empty (default value), the name of the cluster this health check is associated
+    // with will be used.
     string host = 1;
 
     // Specifies the HTTP path that will be requested during health checking. For example