[clang] [rtsan] Update docs to include run-time flags (PR #110296)

[Florian Mayer via cfe-commits]

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@@ -84,6 +84,75 @@ non-zero exit code.
     #14 0x0001958960dc  (<unknown module>)
     #15 0x2f557ffffffffffc  (<unknown module>)
 
+Run-time flags
+--------------
+
+RealtimeSanitizer supports a number of run-time flags, which can be specified in the ``RTSAN_OPTIONS`` environment variable:
+
+.. code-block:: console
+
+   % RTSAN_OPTIONS=option_1=true:path_option_2="/some/file.txt" ./a.out
+   ...
+
+Or at compile-time by overloading the symbol ``__rtsan_default_options``:
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fmayer wrote:

should we just add `__attribute__((__visibility__("default")))` for good measure? because i think if someone adds `-fvisibility=hidden` it won't work without this?

https://github.com/llvm/llvm-project/pull/110296