Add clarification as to error handling with gNMI extensions.

[robshakir]

 * (M) rpc/gnmi/gnmi-extensions.md
  - Add clarification that unknown extensions must not be treated
    as an error.
  - Add clarification that known extensions that are invalid may
    be treated as an error.

[robshakir]

@karimra @limehat @dplore @ncorran @earies @aaronbee PTAL

[LimeHat]

Lgtm. Thanks.