[clang] docs(clang): add some missing flags (PR #209519)

[Balázs Benics via cfe-commits]

================
@@ -349,6 +349,28 @@ output format of the diagnostics that it generates.
        ::
 
            t.c +3:11: warning: conversion specifies type 'char *' but the argument has type 'int'
+
+   **sarif**
+       Emit diagnostics as a `SARIF <https://sarifweb.azurewebsites.net/>`_ JSON document.
+       This is useful when diagnostics are consumed by tools, including AI agents.
+       SARIF diagnostics are written to standard error.
----------------
steakhal wrote:

Are you sure about this? I'm pretty sure stderr shouldn't be the channel for output. If anything, it could be stdout.

https://github.com/llvm/llvm-project/pull/209519