SafeStack documentation improvements

This patch makes the following improvements to the SafeStack documentation:

Explicitly states the security guarantees of the SafeStack
Clarifies which of the security guarantees are probabilistic
Re-orders security limitations to put the most severe ones first
Explains how `__attribute__((no_sanitize("safe-stack")))` works and how to use it safely
Explains that SafeStack should be combined with a forward-edge protection mechanism, such as CPI, IFCC or others
Multiple readability and stylistic improvements

Patch by Volodymyr Kuznetsov!

Differential Revision: http://reviews.llvm.org/D10598

llvm-svn: 240472