[Bug libstdc++/68792] Review doxygen output and don't install useless things

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68792

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #2)
> This config setting would be a good start:
> 
> EXCLUDE_SYMBOLS        = std::_[A-Z]*

Hmm, I forgot about that setting. I've been adding @cond and @endcond around
impl details to suppress them instead.