[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

clang/docs/ClangCommandLineReference.rst

@@ -2136,7 +2136,7 @@ Enable stack clash protection
 
 .. option:: -fstack-protector, -fno-stack-protector
 
-Enable stack protectors for some functions vulnerable to stack smashing. This uses a loose heuristic which considers functions vulnerable if they contain a char (or 8bit integer) array or constant sized calls to alloca, which are of greater size than ssp-buffer-size (default: 8 bytes). All variable sized calls to alloca are considered vulnerable
+Enable stack protectors for some functions vulnerable to stack smashing. This uses a loose heuristic which considers functions vulnerable if they contain a char (or 8bit integer) array or constant sized calls to alloca , which are of greater size than ssp-buffer-size (default: 8 bytes). All variable sized calls to alloca are considered vulnerable. A function witha stack protector has a guard value added to the stack frame that is checked on function exit. The guard value must be positioned in the stack frame such that a buffer overflow from a vulnerable variable will overwrite the guard value before overwriting the function's return address. The reference stack guard value is stored in a global variable.
 
 .. option:: -fstack-protector-all
 

clang/include/clang/Driver/Options.td

@@ -1801,10 +1801,15 @@ def fstack_protector_strong : Flag<["-"], "fstack-protector-strong">, Group<f_Gr
            "as well as any calls to alloca or the taking of an address from a local variable">;
 def fstack_protector : Flag<["-"], "fstack-protector">, Group<f_Group>,
   HelpText<"Enable stack protectors for some functions vulnerable to stack smashing. "
-           "This uses a loose heuristic which considers functions vulnerable "
+           "This uses a loose heuristic which considers functions vulnerable if they "
-           "if they contain a char (or 8bit integer) array or constant sized calls to "
+           "contain a char (or 8bit integer) array or constant sized calls to alloca "
-           "alloca, which are of greater size than ssp-buffer-size (default: 8 bytes). "
+           ", which are of greater size than ssp-buffer-size (default: 8 bytes). All "
-           "All variable sized calls to alloca are considered vulnerable">;
+           "variable sized calls to alloca are considered vulnerable. A function with"
+           "a stack protector has a guard value added to the stack frame that is "
+           "checked on function exit. The guard value must be positioned in the "
+           "stack frame such that a buffer overflow from a vulnerable variable will "
+           "overwrite the guard value before overwriting the function's return "
+           "address. The reference stack guard value is stored in a global variable.">;
 def ftrivial_auto_var_init : Joined<["-"], "ftrivial-auto-var-init=">, Group<f_Group>,
   Flags<[CC1Option, CoreOption]>, HelpText<"Initialize trivial automatic stack variables: uninitialized (default)"
   " | pattern">, Values<"uninitialized,pattern">;