Update docs for _CCCL_IF_CONSTEVAL (#6692)

Author: davebayer

docs/cccl/development/macro.rst

@@ -330,6 +330,20 @@ Usage example:
 | ``_CCCL_PRAGMA_NOUNROLL()``    | Portable ``#pragma nounroll`` pragma      |
 +--------------------------------+-------------------------------------------+
 
+**Conditional Constant Evaluation Macros**
+
+In C++23, the ``if consteval`` statement (`link <https://en.cppreference.com/w/cpp/language/if.html>`_) was introduced. CCCL mimics the behaviour with a set of macros that expand to an implementation supported by the compiler. If the compiler doesn't support any kind of conditional constant evaluation, the macros expand to predefined fallback values.
+
++------------------------------------+-----------------------------------------------------------------------------------+
+| ``_CCCL_IF_CONSTEVAL``             | Equivalent to ``if consteval`` statement (fallbacks to ``if constexpr (false)``)  |
++------------------------------------+-----------------------------------------------------------------------------------+
+| ``_CCCL_IF_CONSTEVAL_DEFAULT``     | Equivalent to ``if consteval`` statement (fallbacks to ``if constexpr (true)``)   |
++------------------------------------+-----------------------------------------------------------------------------------+
+| ``_CCCL_IF_NOT_CONSTEVAL``         | Equivalent to ``if !consteval`` statement (fallbacks to ``if constexpr (true)``)  |
++------------------------------------+-----------------------------------------------------------------------------------+
+| ``_CCCL_IF_NOT_CONSTEVAL_DEFAULT`` | Equivalent to ``if !consteval`` statement (fallbacks to ``if constexpr (false)``) |
++------------------------------------+-----------------------------------------------------------------------------------+
+
 **Exception Macros**
 
 CUDA doesn't support exceptions in device code, however, sometimes we need to write host/device functions that use exceptions on host and ``__trap()`` on device. CCCL provides a set of macros that should be used in place of the standard C++ keywords to make the code compile in both, host and device code.