Resolve "Updating Slack Notification not working by API"

What does this MR do?

• New parameters for for chat notification services are accepted by the services API

• Some cosmetic updates to Service API documentation

• Add some services missing from the Service API Docs

Are there points in the code the reviewer needs to double check?

Tests. Currently the services tests are very generic. Should further tests be added for these specialised services?

Why was this MR needed?

It's not currently possible to perform the same level of configuration of Chat Notification services via the API as it is through the web interface

Some services were undocumented here

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #32364 (moved) Closes #22306 (moved)

Relates to https://gitlab.com/gitlab-org/gitlab-ee/issues/1120