doc: optimize the comment of the Request.Done method (#1454)

README.md

@@ -320,6 +320,23 @@ with fasthttp support:
   fasthttp.ListenAndServe(":80", m)
   ```
 
+* Because creating a new channel for every request is just too expensive, so the channel returned by RequestCtx.Done() is only closed when the server is shutting down.
+
+  ```go
+  func main() {
+	fasthttp.ListenAndServe(":8080", fasthttp.TimeoutHandler(func(ctx *fasthttp.RequestCtx) {
+		select {
+		case <-ctx.Done():
+			// ctx.Done() is only closed when the server is shutting down.
+			log.Println("context cancelled")
+			return
+		case <-time.After(10 * time.Second):
+			log.Println("process finished ok")
+		}
+	}, time.Second*2, "timeout"))
+  }
+  ```
+
 * net/http -> fasthttp conversion table:
 
   * All the pseudocode below assumes w, r and ctx have these types:

server.go

@@ -2697,6 +2697,9 @@ func (ctx *RequestCtx) Deadline() (deadline time.Time, ok bool) {
 // Done returns a channel that's closed when work done on behalf of this
 // context should be canceled. Done may return nil if this context can
 // never be canceled. Successive calls to Done return the same value.
+//
+// Note: Because creating a new channel for every request is just too expensive, so
+// RequestCtx.s.done is only closed when the server is shutting down
 func (ctx *RequestCtx) Done() <-chan struct{} {
 	return ctx.s.done
 }
@@ -2707,6 +2710,9 @@ func (ctx *RequestCtx) Done() <-chan struct{} {
 // If Done is closed, Err returns a non-nil error explaining why:
 // Canceled if the context was canceled (via server Shutdown)
 // or DeadlineExceeded if the context's deadline passed.
+//
+// Note: Because creating a new channel for every request is just too expensive, so
+// RequestCtx.s.done is only closed when the server is shutting down
 func (ctx *RequestCtx) Err() error {
 	select {
 	case <-ctx.s.done: