[Bug libstdc++/40380] class documentation should mention include file to use

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=40380

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
My doxygen patch was merged, so we can start to use SHOW_HEADERFILE and
@headerfile to do this.