Update docs on Context.Done(), Context.Deadline() and Context.Err() (#2196)

Co-authored-by: Bo-Yi Wu <appleboy.tw@gmail.com>

context.go

@@ -1032,26 +1032,20 @@ func (c *Context) SetAccepted(formats ...string) {
 /***** GOLANG.ORG/X/NET/CONTEXT *****/
 /************************************/
 
-// Deadline returns the time when work done on behalf of this context
-// should be canceled. Deadline returns ok==false when no deadline is
-// set. Successive calls to Deadline return the same results.
+// Deadline always returns that there is no deadline (ok==false),
+// maybe you want to use Request.Context().Deadline() instead.
 func (c *Context) Deadline() (deadline time.Time, ok bool) {
 	return
 }
 
-// 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.
+// Done always returns nil (chan which will wait forever),
+// if you want to abort your work when the connection was closed
+// you should use Request.Context().Done() instead.
 func (c *Context) Done() <-chan struct{} {
 	return nil
 }
 
-// Err returns a non-nil error value after Done is closed,
-// successive calls to Err return the same error.
-// If Done is not yet closed, Err returns nil.
-// If Done is closed, Err returns a non-nil error explaining why:
-// Canceled if the context was canceled
-// or DeadlineExceeded if the context's deadline passed.
+// Err always returns nil, maybe you want to use Request.Context().Err() instead.
 func (c *Context) Err() error {
 	return nil
 }