doc: add docs for west spdx

These are based on the commit log in
fd31b9b4acb5d48f83ef8cf0f3afe3e48b7844c0.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
diff --git a/scripts/west_commands/spdx.py b/scripts/west_commands/spdx.py
index e06ffec..58e79f8 100644
--- a/scripts/west_commands/spdx.py
+++ b/scripts/west_commands/spdx.py
@@ -32,6 +32,8 @@
                 help=self.help,
                 description = self.description)
 
+        # If you update these options, make sure to keep the docs in
+        # doc/guides/west/zephyr-cmds.rst up to date.
         parser.add_argument('-i', '--init', action="store_true",
                 help="initialize CMake file-based API")
         parser.add_argument('-d', '--build-dir',