Clarification on POST | webrtc-registration

[teikuran]

Problem description
The current specification provides clear guidance for the PUT operation, stating it should be used to refresh a registration before it expires. However, the guidance for POST is less specific, simply stating it is used to "Create a registration".

This creates ambiguity. For example:

• Should a consumer only POST after a previous registration has expired or been terminated (e.g., after receiving a registration-ends event)?
• Is it permissible to POST while a previous registration is still active?

Expected action
Add informational descriptions to clarify the intended use cases for POST.