[DOC] - Clarify the 3 types of REST API token generation methods, maybe also rename them.

[niravcodes]

Describe the requested update

There are 3 ways of creating tokens with the Fabric APIs. Four if we count the OAuth2 token as separate. They aren't described in any particular detail, and it's easy to confuse one for the other. Some also do compound tasks of both creating a subscriber and then sending their token.

Reason for the request
It is confusing.

Sections to be updated

https://developer.signalwire.com/rest/signalwire-rest/endpoints/fabric/subscriber-tokens-create

https://developer.signalwire.com/rest/signalwire-rest/endpoints/fabric/subscribers-tokens

Additional context

https://signalwire.slack.com/archives/C07AVG5G6SW/p1745581481023279

(Slack chat conversation with @imfaruk)