[Docs] Expand -fstack-protector and -fstack-protector-all

Improve the description of these command line options
by providing specific heuristic information, as outlined
for the ssp function attribute(s) in LLVM's documentation.

Also rewords -fstack-protector-all for affinity.

Differential Revision: https://reviews.llvm.org/D55428

llvm-svn: 349335