Add missing doxygen style comments to XXH3 family

[t-mat]

Trivial PR which adds basic Doxygen style comment to XXH3 family funcions.

Hopefully we'll be able to introduce new CI test for documentation.

[t-mat]

When I change the following line in the Doxyfile, doxygen still give me a several hundred warnings 😢

        # Until we document everything
-       WARN_IF_UNDOCUMENTED   = NO
+       WARN_IF_UNDOCUMENTED   = YES

It seems it'll need more PRs. So, I think we won't achieve this goal at 0.8.2 timely.

[Cyan4973]

Understandable, and fully acceptable outcome.

[Cyan4973]

I was wondering, regarding doxygen documentation,
what's the better policy ?
Should it be pre-generated inside the repository, typically before a release,
or is it better to let repository users generate it on their side ?
(another option could be to provide it as a release artifact)

[t-mat]

I think almost all open source project doesn't provide output of doxygen (or other document generators) as a part of release (or their repository).
Instead of it, they provides the document as web page.

For example, FLAC project releases its doxygen-generated document as https://xiph.org/flac/api/index.html

Another example is SDL. They have doxyfile. But they don't provide its output as part of their release.

[Cyan4973]

Thanks @t-mat , these are very good references.