Registration API - Inline documentation expanded

[stroncoso-quobis]

What type of PR is this?

Add one of the following kinds:

• documentation

What this PR does / why we need it:

• Commonalities: API documentation must be included inline
• Terms and definitions, like clientId and deviceId must be clarified
• No clear introductory information for newcomers

Which issue(s) this PR fixes:

Part of #60 , #63 and #50

Special notes for reviewers:

Information included was aligned with PR#62 (release review) comments and working team discussions.
It was removed channelId as agreed at PR#65

Changelog input

doc(registration): Expanded API inline documentation
fix(registration): removed channelId property
doc(registration): Authorization, and extra API info

Additional documentation

n/a