[DOCS] Add more detail to stack protector documentation

The Clang -fstack-protector documentation mentions what functions are considered
vulnerable but does not mention the details of the implementation such as the use
of a global variable for the guard value. This brings the documentation more in
line with the GCC documentation at:
https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html
and gives someone using the option more idea about what is protected.

This partly addresses https://bugs.llvm.org/show_bug.cgi?id=42764

Differential Revision: https://reviews.llvm.org/D85239