Specify that mesh gateways require Envoy (#6506)

* Specify that mesh gateways must operate on L7

* Add feedback from Matt

* clarify gateway requirements

website/source/docs/connect/mesh_gateway.html.md

@@ -36,6 +36,24 @@ Mesh gateways also require that your Consul datacenters are configured correctly
 - [gRPC](/docs/agent/options.html#grpc_port) must be enabled.
 - If you want to [enable gateways globally](/docs/connect/mesh_gateway.html#enabling-gateways-globally) you must enable [centralized configuration](/docs/agent/options.html#enable_central_service_config).
 
+Currently, Envoy is the only proxy with mesh gateway capabilities in Consul.
+
+- Mesh gateway proxies receive their configuration through Consul, which
+automatically generates it based on the proxy's registration. Currently Consul
+can only translate mesh gateway registration information into Envoy
+configuration, therefore the proxies acting as mesh gateways must be Envoy.
+
+- Sidecar proxies that send traffic to an upstream service through a gateway
+need to know the location of that gateway. They discover the gateway based on
+their sidecar proxy registrations. Consul can only translate the gateway
+registration information into Envoy configuration, so any sidecars that send
+upstream traffic through a gateway must be Envoy.
+
+Sidecar proxies that don't send upstream traffic through a gateway aren't
+affected when you deploy gateways. If you are using Consul's built-in proxy as a
+Connect sidecar it will continue to work for intra-datacenter traffic and will
+receive incoming traffic even if that traffic has passed through a gateway.
+
 ## Modes of Operation
 
 Each upstream of a Connect proxy can be configured to be routed through a mesh gateway. Depending on