Help: Document CMAKE_TRY_COMPILE_PLATFORM_VARIABLES example

Suggested-by: Hendrik Sattler <post@hendrik-sattler.de>

Help/variable/CMAKE_TRY_COMPILE_PLATFORM_VARIABLES.rst

@@ -8,3 +8,19 @@ the host project.
 This variable should not be set by project code.  It is meant to be set by
 CMake's platform information modules for the current toolchain, or by a
 toolchain file when used with :variable:`CMAKE_TOOLCHAIN_FILE`.
+
+Variables meaningful to CMake, such as :variable:`CMAKE_<LANG>_FLAGS`, are
+propagated automatically.  The ``CMAKE_TRY_COMPILE_PLATFORM_VARIABLES``
+variable may be set to pass custom variables meaningful to a toolchain file.
+For example, a toolchain file may contain:
+
+.. code-block:: cmake
+
+  set(CMAKE_SYSTEM_NAME ...)
+  set(CMAKE_TRY_COMPILE_PLATFORM_VARIABLES MY_CUSTOM_VARIABLE)
+  # ... use MY_CUSTOM_VARIABLE ...
+
+If a user passes ``-DMY_CUSTOM_VARIABLE=SomeValue`` to CMake then this
+setting will be made visible to the toolchain file both for the main
+project and for test projects generated by the :command:`try_compile`
+command source file signature.