Update Knative IP address docs

What does this MR do?

Since we now load the IP address automatically for you per https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23162 we no longer need to mention that it can only be fetched manually. Also since Knative/Serverless features are still in Alpha I did not feel it was necessary to document that this was only available after a certain version.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• Documentation created/updated
• [-] Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.