Help: Add missing gate for project(SPDX_LICENSE)

The `SPDX_LICENSE` option of the `project` command requires that the
`CMAKE_EXPERIMENTAL_EXPORT_PACKAGE_INFO` gate is enabled, but this was
not documented. Add the missing documentation.

Fixes: #27328

Help/command/project.rst

@@ -109,6 +109,9 @@ The options are:
 
 ``SPDX_LICENSE <license-string>``
   .. versionadded:: 4.2
+  .. note::
+
+    Experimental. Gated by ``CMAKE_EXPERIMENTAL_EXPORT_PACKAGE_INFO``.
 
   Optional.
   Sets the variables