Fix formatting conditionals with comments

[sdankel]

Description

Closes #4935

• In order to write comments for the else block when it only contains a comment, I had to add the span to CodeBlockContents

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.

[sdankel]

I'm not sure why the markdown lint is failing...