doc/netlink: Update schema to support cmd-cnt-name and cmd-max-name

allow specifying cmd-cnt-name and cmd-max-name in netlink specs, in
accordance with Documentation/userspace-api/netlink/c-code-gen.rst.

Use cmd-cnt-name and attr-cnt-name in the mptcp yaml spec and in the
corresponding uAPI headers, to preserve the #defines we had in the past
and avoid adding new ones.

v2:
 - squash modification in mptcp.yaml and MPTCP uAPI headers

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Link: https://lore.kernel.org/r/12d4ed0116d8883cf4b533b856f3125a34e56749.1698415310.git.dcaratti@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/netlink/specs/mptcp.yaml

@@ -8,6 +8,7 @@ c-family-name: mptcp-pm-name
 c-version-name: mptcp-pm-ver
 max-by-define: true
 kernel-policy: per-op
+cmd-cnt-name: --mptcp-pm-cmd-after-last
 
 definitions:
   -
@@ -167,6 +168,7 @@ attribute-sets:
   -
     name: attr
     name-prefix: mptcp-pm-attr-
+    attr-cnt-name: --mptcp-attr-after-last
     attributes:
       -
         name: unspec