[clang] [Docs] Some updates to the Clang user's manual (PR #151702)

[Aaron Ballman via cfe-commits]

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@@ -36,7 +36,7 @@ language-specific information, please see the corresponding language
 specific section:
 
 -  :ref:`C Language <c>`: K&R C, ANSI C89, ISO C90, ISO C94 (C89+AMD1), ISO
-   C99 (+TC1, TC2, TC3).
+   C99 (+TC1, TC2, TC3), C11, C17, C23, and C2y.
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AaronBallman wrote:

Sure!

https://github.com/llvm/llvm-project/pull/151702