Update documentation and describe "Report Interval" config in Cisco FND

[ismilak]

Currently there is only one config value to set metrics interval in FND. This value is sent by FND to the Agent but it's used for trickle timer. The value is used as imin=imax argument for trickle timer start API.

If the randomisation is intended for metrics sending too, the appropirate documentation update required also: since metrics sending happens in randomised interval, but there is an exact config value in Cisco FND.