CheckLanguage: Document that CMAKE_CUDA_HOST_COMPILER should not be set alone

Follow up commit 7968d9d442 (Help: Document CMAKE_CUDA_HOST_COMPILER as
specific to NVIDIA nvcc, 2020-07-14, v3.19.0-rc1~501^2) with an explicit
note in `CheckLanguage`.

Issue: #25093

Modules/CheckLanguage.cmake

@@ -33,6 +33,14 @@ or :command:`project` commands:
     If the check detects that no explicit host compiler is needed,
     this variable will be cleared.
 
+    If this variable is already set, its value is preserved only if
+    :variable:`CMAKE_CUDA_COMPILER <CMAKE_<LANG>_COMPILER>` is also set.
+    Otherwise, the check runs and overwrites
+    :variable:`CMAKE_CUDA_HOST_COMPILER` with a new result.
+    Note that :variable:`CMAKE_CUDA_HOST_COMPILER` documents it should
+    not be set without also setting
+    :variable:`CMAKE_CUDA_COMPILER <CMAKE_<LANG>_COMPILER>` to a NVCC compiler.
+
 For example:
 
 .. code-block:: cmake