better error message for doc comment without an item

[xunilrj]

Description

This PR closes #4968. The problem here is that doc comments are special in the sense that they demand an item after.

I am following the rustc error message of my machine, but rust playground message is a little different. Not sure which way we want to go.

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=34097e067111a355e70767be7965952c

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.