Improve help for code blocks invalid attributes

[GuillaumeGomez]

In #71408, we added a new lint to warn user in case of invalid (but close to existing) attributes. However, we say that "it might not be recognized as a rust block or...". This could be improved if we knew for sure that it is a rust block before actually throwing the message. The problem currently is that the order of the attributes actually matters, which makes it a bit trickier than one might expect.

[GuillaumeGomez]

I'm not sure there is much that can be done here. Closing.