Help: Clarify cmake_host_system_information OS_NAME

Fixes: #26631

Help/command/cmake_host_system_information.rst

@@ -129,7 +129,15 @@ queried.  The list of queried values is stored in ``<variable>``.
 ``OS_NAME``
   .. versionadded:: 3.10
 
-  See :variable:`CMAKE_HOST_SYSTEM_NAME`
+  The host operating system name:
+
+  * On UNIX platforms, this is ``uname -s``.
+
+  * On Apple platforms, this is ``sw_vers -productName``.
+
+  * On Windows, this is ``Windows``.
+
+  See also :variable:`CMAKE_HOST_SYSTEM_NAME`.
 
 ``OS_RELEASE``
   .. versionadded:: 3.10