netlink: specs: finish up operation enum-models

I had a (bright?) idea of introducing the concept of enum-models
to account for all the weird ways families enumerate their messages.
I've never finished it because generating C code for each of them
is pretty daunting. But for languages which can use ID values directly
the support is simple enough, so clean this up a bit.

"unified" model is what I recommend going forward.
"directional" model is what ethtool uses.
"notify-split" is used by the proposed DPLL code, but we can just
make them use "unified", it hasn't been merged :)

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

Documentation/netlink/genetlink.yaml

@@ -188,9 +188,7 @@ properties:
           to a single enum.
           "directional" has the messages sent to the kernel and from the kernel
           enumerated separately.
-          "notify-split" has the notifications and request-response types in
-          different enums.
-        enum: [ unified, directional, notify-split ]
+        enum: [ unified ]
       name-prefix:
         description: |
           Prefix for the C enum name of the command. The name is formed by concatenating