Refactor the only and except complex section

What does this MR do?

Mainly documents the logic behind the only and except keywords based on the chat discussion https://gitlab.slack.com/archives/C02PF508L/p1544619113366900.

Also, took the time to refactor the relevant section. Specifically:

• Split up refs and kubernetes to their own section
• Use only and except in all headings explicitly
• Document the boolean logic of only and except

Author's checklist

• Apply the correct labels and milestone
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8837

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8837