[Bug driver/69849] Some spec functions are undocumented in doc/invoke.texi

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69849

--- Comment #1 from vries at gcc dot gnu.org ---
In addition, the '%{%:function(args):X}' construct is undocumented, as noted
here: https://gcc.gnu.org/ml/gcc-patches/2016-02/msg01243.html .