I've discussed this with a number of people during the last days, and
here are my thoughts:

- I'm not convinced that the status quo is so good that we don't need
to bother with standardization. Many tools support doc comments one
way or the other, but such support is often limited and inconsistent.
IntelliSense displays Doxygen comments in tooltips that clangd
doesn't, for example.

- A whitepaper by the Tooling SG seems like the right feature for
this. It could be part of the core language spec if it had semantic
effects, like @deprecated tags behaving equivalently to
[[deprecated]], but I think that's too controversial anyway.

- Doxygen is a quasi-standard, but has tons of tags with varying
support from other tooling. The goal would be to define a common
syntax and set of doc comment tags that we expect all documentation
tools (that care) to support. It's also not always clear to which
construct a doc comment can be used.

- I don't think that Markdown is a fad. It's definitely a good tool
for this job and isn't going away anytime soon. If we were to support
Markdown, there exists the CommonMark standard. Anything beyond that
would be non-standard. It's not clear that we need to specify what the
contents of doc comments are anyway. We could just standardize the
basic syntax, which C++ constructors the comments apply to, and what
tags inside are supported. Making them display pretty could then be
QoI.

- Regarding syntax: the best bet is probably to standardize "///" and
"/**" as doc comments, and Javadoc-style "@param" etc. These are the
greatest common denominator for tooling.

- TODO, FIXME, etc. aren't doc comments. NOLINT, clang-format off,
etc. are all outside the scope of such standardization in my opinion,
or at least, it would make it much harder to standardize if we don't
focus on a specific issue, like just doc comments.