recommendation to use only line comments not justified

[brson]

It just says to use line comments but not why.

[nathantypanski]

The justification, IMO, is that:

• Line comments are easy to spot compared to block comments (barring syntax highlighting, but we shouldn't assume everyone has perfect syntax highlighting always - especially in, e.g., a code diff).
• Line comments make it easier to keep consistent indentation. This is especially relevant in Markdown documentation, which is indentation-aware.

Though I think the choice is mostly arbitrary; we're forced to "pick one and make it a convention;" the Google C++ Style Guide remarks that:

Use either the // or /* */ syntax, as long as you are consistent.

You can use either the // or the /* */ syntax; however, // is much more common. Be consistent with how you comment and what style you use where.

The argument that "// is much more common in Rust" (much like above) is IMO the best argument in its favor, however. When it comes to code style, "my way is popular" is the most important justification you could have.

[aturon]

The pure style (non-API) part of the guidelines need a complete overhaul and RFC laying out the rationale, IMO. But no one has had the time to do this yet.