Access token security considerations for Number Verification API

[jpengar]

What type of PR is this?

• enhancement/feature

What this PR does / why we need it:

This pull request updates the documentation for the Number Verification API to clarify and strengthen its authentication requirements. The main focus is on enhancing security by specifying stricter rules for access tokens used in the verification process.

Authentication and Security Enhancements:

• Added explicit requirements that access tokens for number verification must be single-use, preventing replay attacks and ensuring each token is valid for only one successful API call.
• Stated that refresh tokens must not be issued for number verification scopes; a new authorization flow is required for each new access token.
• Limited the maximum expiration time for access tokens to 300 seconds (5 minutes) to reduce the risk window for token misuse.

Which issue(s) this PR fixes:

Fixes #154

Special notes for reviewers:

N/A

Changelog input

 access token security requirements

Additional documentation

N/A

[bigludo7]

LGTM