doc: ynl: Add doc attr to struct members in genetlink-legacy spec

Make it possible to document the meaning of struct member attributes in
genetlink-legacy specs.
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/netlink/genetlink-legacy.yaml

@@ -124,6 +124,9 @@ properties:
                 $ref: '#/$defs/len-or-define'
               byte-order:
                 enum: [ little-endian, big-endian ]
+              doc:
+                description: Documentation for the struct member attribute.
+                type: string
         # End genetlink-legacy
 
   attribute-sets: