Help: Clarify valid versus supported values for CUDA_STANDARD

CMake 3.18 added the first support for any compiler for 17 and 20,
but those were recognized as valid values in earlier CMake versions
even though there was no compiler that supported them. Make this
distinction clear to avoid creating the impression that these standards
could be usefully used before CMake 3.18.

While 98 is recognized as a valid value, it also just gets treated as 03
internally. Document this behavior as well.

Fixes: #22711

Help/prop_tgt/CUDA_STANDARD.rst

@@ -12,21 +12,27 @@ flag such as ``-std=gnu++11`` to the compile line.
 Supported values are:
 
 ``98``
-  CUDA C++98
+  CUDA C++98. Note that this maps to the same as ``03`` internally.
+
+``03``
+  CUDA C++03
 
 ``11``
   CUDA C++11
 
 ``14``
-  CUDA C++14
+  CUDA C++14. While CMake 3.8 and later *recognize* ``14`` as a valid value,
+  CMake 3.9 was the first version to include support for any compiler.
 
 ``17``
-  CUDA C++17
+  CUDA C++17. While CMake 3.8 and later *recognize* ``17`` as a valid value,
+  CMake 3.18 was the first version to include support for any compiler.
 
 ``20``
   .. versionadded:: 3.12
 
-  CUDA C++20
+  CUDA C++20. While CMake 3.12 and later *recognize* ``20`` as a valid value,
+  CMake 3.18 was the first version to include support for any compiler.
 
 ``23``
   .. versionadded:: 3.20
@@ -43,7 +49,7 @@ means that using:
 
 with a compiler which does not support ``-std=gnu++11`` or an equivalent
 flag will not result in an error or warning, but will instead add the
-``-std=gnu++98`` flag if supported.  This "decay" behavior may be controlled
+``-std=gnu++03`` flag if supported.  This "decay" behavior may be controlled
 with the :prop_tgt:`CUDA_STANDARD_REQUIRED` target property.
 Additionally, the :prop_tgt:`CUDA_EXTENSIONS` target property may be used to
 control whether compiler-specific extensions are enabled on a per-target basis.