[Bug c/96907] [docs] document builtins for fputc and putc

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96907

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
They are not really "builtins", they are merely present to have default
attributes attached to them.