[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
+prepended to.
+
+__auto_type
+-----------
+
+``__auto_type`` behaves the same as C++11s ``auto`` but is available in all
+language modes.
+
+__imag, __imag__
+----------------
+
+``__imag`` and ``__imag__`` can be used to get the imaginary part of a complex
+value.
+
+__real, __real__
+----------------
+
+``__real`` and ``__real__`` can be used to get the real part of a complex value.
+
+__asm, __asm__
+--------------
+
+``__asm`` and ``__asm__`` are alternate spellings for ``asm``, but available in
+all language modes.
+
+__complex, __complex__
+----------------------
+
+``__complex`` and ``__complex__`` are alternate spellings for ``_Complex``.
+
+__const, __const__
+------------------
+
+``__const`` and ``__const__`` are alternate spellings for ``const``.
+
+__decltype
+----------
+
+``__decltype`` is an alternate spelling for ``decltype``, but is also available
+in C++ modes before C++11.
+
+__inline, __inline__
+--------------------
+
+``__inline`` and ``__inline__`` are alternate spellings for ``inline``, but are
+available in all language modes.
+
+__nullptr
+---------
+
+``__nullptr`` is an alternate spelling for ``nullptr``, but is also available in
+C++ modes before C++11.
+
+__restrict, __restrict__
+------------------------
+
+``__restrict`` and ``__restrict__`` are alternate spellings for ``restrict``,
+but are available in all language modes.
+
+__signed, __signed__
----------------
AaronBallman wrote:

Should probably mention that we do not support `__unsigned__`, which certainly surprised me (neither does GCC)

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