Clarifying the parameters section -- defaults, allowed values, deactivation

[swcurran]

Update the section on parameters (3.7.1 to clarify:

• default values when parameters are introduced, but not set by the DID Controller.
  • Happens in the first log entry, and in log entries when a DID Controller updates to an new spec version that adds new parameters.
• Allowed values
• How to "deactivate" a parameter (if appropriate).

A general piece of guidance is that the JSON null value must not be used, as its use removes the typing information from the parameter. Thanks to @dmitrizagidulin for that excellent guidance.

This was discussed at the 2025.04.03 did:webvh DID Method Work Item Meeting (Agenda, Recording)