[Bug c++/89244] __builtin_is_constant_evaluated not documented

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89244

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Well we do document some of them:
https://gcc.gnu.org/onlinedocs/gcc/Type-Traits.html

I'm fine not doing so though.