[clang] [Clang] Document some of the implementation-defined keywords (PR #84591)

[Aaron Ballman via cfe-commits]

================
@@ -433,6 +433,117 @@ __datasizeof
 ``__datasizeof`` behaves like ``sizeof``, except that it returns the size of the
 type ignoring tail padding.
 
+_BitInt, _ExtInt
+----------------
+
+Clang supports the C23 ``_BitInt(N)`` feature as an extension in older C modes
+and in C++. This type was previously implemented in Clang with the same
+semantics, but spelled ``_ExtInt(N)``. This spelling has been deprecated in
+favor of the standard type.
+
+Note: the ABI for ``_BitInt(N)`` is still in the process of being stabilized,
+so this type should not yet be used in interfaces that require ABI stability.
+
+C keywords supported in all language modes
+------------------------------------------
+
+Clang supports ``_Alignas``, ``_Alignof``, ``_Atomic``, ``_Complex``,
+``_Generic``, ``_Imaginary``, ``_Noreturn``, ``_Static_assert``,
+``_Thread_local``, ``_Float16``, ``_Decimal32``, ``_Decimal64`` and
+``_Decimal128`` in all language modes with the C semantics.
+
+__alignof, __alignof__
+----------------------
+
+``__alignof`` and ``__alignof__`` return, in contrast to ``_Alignof`` and
+``alignof``, the preferred alignment of a type. This may be larger than the
+required alignment for improved performance.
+
+__extension__
+-------------
+
+``__extension__`` suppressed extension diagnostics in the statement it is
----------------
AaronBallman wrote:

```suggestion
``__extension__`` suppresses extension diagnostics in the statement it is
```

https://github.com/llvm/llvm-project/pull/84591