Clarify election behaviours.
Clarify election behaviours.

 * (M) v1/proto/service/gribi.proto
  - Clarify that when a non-primary client sends a ModifyRequest then
    it should receive an explicit `FAILED` response.
  - Clarify that when the election ID sent by a client is equal to the
    previous highest, the new client becomes the master.

openconfig/gribi