Update ServerInHandle comments (#1437)

* Update ServerInHandle comments

* Include code string not code number for RST_STREAM

tap/tap.go

@@ -32,8 +32,20 @@ type Info struct {
 	// TODO: More to be added.
 }
 
-// ServerInHandle defines the function which runs when a new stream is created
-// on the server side. Note that it is executed in the per-connection I/O goroutine(s) instead
-// of per-RPC goroutine. Therefore, users should NOT have any blocking/time-consuming
-// work in this handle. Otherwise all the RPCs would slow down.
+// ServerInHandle defines the function which runs before a new stream is created
+// on the server side. If it returns a non-nil error, the stream will not be
+// created and a RST_STREAM will be sent back to the client with REFUSED_STREAM.
+// The client will receive an RPC error "code = Unavailable, desc = stream
+// terminated by RST_STREAM with error code: REFUSED_STREAM".
+//
+// It's intended to be used in situations where you don't want to waste the
+// resources to accept the new stream (e.g. rate-limiting). And the content of
+// the error will be ignored and won't be sent back to the client. For other
+// general usages, please use interceptors.
+//
+// Note that it is executed in the per-connection I/O goroutine(s) instead of
+// per-RPC goroutine. Therefore, users should NOT have any
+// blocking/time-consuming work in this handle. Otherwise all the RPCs would
+// slow down. Also, for the same reason, this handle won't be called
+// concurrently by gRPC.
 type ServerInHandle func(ctx context.Context, info *Info) (context.Context, error)