Fix Command.Context comment (#1639)

command.go

@@ -224,8 +224,13 @@ type Command struct {
 	SuggestionsMinimumDistance int
 }
 
-// Context returns underlying command context. If command wasn't
-// executed with ExecuteContext Context returns Background context.
+// Context returns underlying command context. If command was executed
+// with ExecuteContext or the context was set with SetContext, the
+// previously set context will be returned. Otherwise, nil is returned.
+//
+// Notice that a call to Execute and ExecuteC will replace a nil context of
+// a command with a context.Background, so a background context will be
+// returned by Context after one of these functions has been called.
 func (c *Command) Context() context.Context {
 	return c.ctx
 }