tools: ynl: don't append doc of missing type directly to the type

When using YNL in tests appending the doc string to the type
name makes it harder to check that we got the correct error.
Put the doc under a separate key.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20240426003111.359285-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>