Convert comments to doc comments where sensible

[tertsdiepraam]

Using doc comments has a couple of advantages:

• The documentation shows up in rustdoc.
• IDE's show the doc comments on hover.
• Rustdoc can warn when links to items are invalid, which helps with keeping the docs up to date.

Rustdoc works best with this format:

/// <one line summary>
///
/// <longer description>

Because it will then only show the summary in some places, but I haven't changed all comments to this format and left a lot as is.

I also tried adding backticks and brackets for inline code and links, but I've undoubtedly missed a few places.

In the current version, rustdoc will generate a few warnings because I link to a few private items from public items. I'd have to know more about the internals to fix this properly and the documentation still works.

All documentation can be viewed with

cargo doc --document-private-items --open