[clang] [Clang][Docs] Add documentation for -Wfortify-source (PR #224111)

[Nick Desaulniers via cfe-commits]

================
@@ -1897,7 +1897,35 @@ def CrossTURemarks : DiagGroup<"ctu-remarks">;
 
 def CTADMaybeUnsupported : DiagGroup<"ctad-maybe-unsupported">;
 
-def FortifySource : DiagGroup<"fortify-source", [FormatOverflow, FormatTruncation]>;
+def FortifySource : DiagGroup<"fortify-source", [FormatOverflow, FormatTruncation]> {
+  code Documentation = [{
+Warns at compile time when calls to standard C library or POSIX functions have
+provably out-of-bounds destination buffers or invalid constant arguments,
+modeled after [`_FORTIFY_SOURCE`](https://www.gnu.org/software/libc/manual/html_node/Source-Fortification.html)
----------------
nickdesaulniers wrote:

ping @shafik 

In particular, this PR is discussing -Wfortify-source, but it's not describing the _larger and more important (IMO)_ machinery behind `-DFORTIFY_SOURCE={1,2,3}`. That is a complex topic.

https://github.com/llvm/llvm-project/pull/224111