[Bug lto/95190] Documentation for -Wstringop-overflow

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95190

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
This is new behavior for warnings in GCC 10 and now how all other optimization
options behave - the option state is fixed per function at compile-time and
carried over to link-time.

Indeed warning options are not mentioned in the docs, let me fix that.