Parameter Protocol - Add the _HASH check info?

[hamishwillee]

PX4 implements the Parameter Protocol as written, but additionally supports some sort of caching of parameters that have been sent using a special parameter name HASH. I haven't looked at the details, but this should make it easier to inform a GCS that it is up to date without having to resend all parameters.

The requirement is to determine whether this is considered part of the spec, and if so, document it. If not part of spec, should still mention in.