Improve documentation regard questions about code

https://docs.gitlab.com/ee/user/gitlab_duo_chat/examples.html#ask-about-code only offers examples where the question is self-sufficient. However, developers often have questions about the code in the editor. They can ask such question but they have to select the code in the editor before sending the question. We have seen users doing weird workarounds by first selecting the code asking to have it explained to only then ask the question about the code that they actually had.