add comment for context (#1413)

ref #1075 
annotation from go context source.

context.go

@@ -836,18 +836,33 @@ 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.
 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.
 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.
 func (c *Context) Err() error {
 	return nil
 }
 
+// Value returns the value associated with this context for key, or nil
+// if no value is associated with key. Successive calls to Value with
+// the same key returns the same result.
 func (c *Context) Value(key interface{}) interface{} {
 	if key == 0 {
 		return c.Request