doc: develop: west: document spdx not being supported for native_sim

Generating SPDX SBOMs for the ``native_sim`` platform has propably
never worked, but at least now it will be documented.

Signed-off-by: Benjamin Cabé <[email protected]>

Author: kartben

doc/develop/west/zephyr-cmds.rst

@@ -165,6 +165,10 @@ source files that are compiled to generate the built library files.
 - ``--include-sdk``: with ``--analyze-includes``, also create a fourth SPDX
   document, :file:`sdk.spdx`, which lists header files included from the SDK.
 
+.. warning::
+
+   The generation of SBOM documents for the ``native_sim`` platform is currently not supported.
+
 .. _SPDX specification clause 6:
    https://spdx.github.io/spdx-spec/v2.2.2/document-creation-information/