[Bug c/69759] __builtin_alloca undocumented

["pinskia at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69759

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
It is documented, just not the way you think it is:
See https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html:
Outside strict ISO C mode (-ansi, -std=c90, -std=c99 or -std=c11), the
functions _exit, +++alloca+++, bcmp, bzero, dcgettext, dgettext, dremf, dreml,
drem, exp10f, exp10l, exp10, ffsll, ffsl, ffs, fprintf_unlocked,
fputs_unlocked, gammaf, gammal, gamma, gammaf_r, gammal_r, gamma_r, gettext,
index, isascii, j0f, j0l, j0, j1f, j1l, j1, jnf, jnl, jn, lgammaf_r, lgammal_r,
lgamma_r, mempcpy, pow10f, pow10l, pow10, printf_unlocked, rindex, scalbf,
scalbl, scalb, signbit, signbitf, signbitl, signbitd32, signbitd64,
signbitd128, significandf, significandl, significand, sincosf, sincosl, sincos,
stpcpy, stpncpy, strcasecmp, strdup, strfmon, strncasecmp, strndup, toascii,
y0f, y0l, y0, y1f, y1l, y1, ynf, ynl and yn may be handled as built-in
functions. All these functions have corresponding versions prefixed with
__builtin_, which may be used even in strict C90 mode.

+++ added to show it is there.