Remove \fn in Doxygen comments where possible

According to the Doxygen manual, quoted below:

https://doxygen.nl/manual/commands.html#cmdfn

> This command is only needed if a comment block is not placed in front (or behind) the function declaration or definition.
>
> If your comment block is in front of the function declaration or definition this command can (and to avoid redundancy should) be omitted.
> Warning: Do not use this command if it is not absolutely needed, since it will lead to duplication of information and thus to errors.

I think it would improve our existing comments to adopt the standard of omitting redundant \fn commands, which also means removing it wherever possible. I hope to receive feedback on this proposal before any action is taken.

Start by reviewing the linked Doxygen manual section on \fn, then search the repository for \fn commands in documentation comments. Identify which commands are redundant with nearby declarations or definitions; done means the removable instances are handled consistently after the proposal receives maintainer feedback.

Written by the indexing model from the issue text.