chore: adding domains related method documentation

[Gregory-Gerard]

Fixes

Small PR for adding domains related method documentation, to help our IDE with these endpoints (https://docs.sendgrid.com/api-reference/domain-authentication/list-all-authenticated-domains for example). No code changes, just PHPDoc.

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the main branch
• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

[Kingdutch]

Looks like 1.5 years later this info is unfortunately still missing which makes these kinds of tasks more difficult. @SendGridDX What would be needed to get this PR merged? :D