Explicitly specify invalid combinations of fields.

[robshakir]

 * (M) doc/specification.md
 * (M) v1/proto/service/gribi.proto
  - Clarify explicitly that specifying more than one of `election_id`
    `params` and `operation` together is not valid. The specification
    implicitly said this before (`session_params` must be the only
    field in the first message, `election_id` must be the only field
    populated when updating an election ID, etc. This change makes this
    explicit.