docs: gateway.writable removal

- (readonly) is confusing since we have Gateway.NoFetch - better to
  remove it from stdout
- more actionable error message and docs

cmd/ipfs/daemon.go

@@ -804,7 +804,7 @@ func serveHTTPGateway(req *cmds.Request, cctx *oldcmds.Context) (<-chan error, e
 	}
 
 	if writable {
-		log.Fatalf("serveHTTPGateway: Gateway.Writable has been REMOVED. Modern replacement tracked in https://github.com/ipfs/specs/issues/375")
+		log.Fatalf("Support for Gateway.Writable and --writable has been REMOVED. Please remove it from your config file or CLI. Modern replacement tracked in https://github.com/ipfs/specs/issues/375")
 	}
 
 	listeners, err := sockets.TakeListeners("io.ipfs.gateway")
@@ -838,7 +838,7 @@ func serveHTTPGateway(req *cmds.Request, cctx *oldcmds.Context) (<-chan error, e
 
 	// we might have listened to /tcp/0 - let's see what we are listing on
 	for _, listener := range listeners {
-		fmt.Printf("Gateway (readonly) server listening on %s\n", listener.Multiaddr())
+		fmt.Printf("Gateway server listening on %s\n", listener.Multiaddr())
 	}
 
 	cmdctx := *cctx

docs/config.md

@@ -681,7 +681,9 @@ Type: `string` (url)
 
 ### `Gateway.Writable`
 
-**REMOVED**: this option has been removed. We are working on a modern replacement. IPIP can be tracked in [ipfs/specs#375](https://github.com/ipfs/specs/issues/375).
+**REMOVED**: this option no longer available as of [Kubo 0.20](https://github.com/ipfs/kubo/blob/master/docs/changelogs/v0.20.md).
+
+We are working on developing a modern replacement. To support our efforts, please leave a comment describing your use case in [ipfs/specs#375](https://github.com/ipfs/specs/issues/375).
 
 ### `Gateway.PathPrefixes`