[Bug middle-end/111659] document that -Wstrict-flex-arrays depends on -ftree-vrp

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111659

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
That is:
It is more effective when -ftree-vrp is active (the default for -O2 and above)
but a subset of instances are issued even without optimization.