Update documentation for processor batch1

What does this MR do and why?

Iteration 1 for #1022 (comment 970729276) Add documentation to the first batch of processors.

We want to eventually be able to dynamically populate the reactive workflow descriptions in https://about.gitlab.com/handbook/engineering/quality/triage-operations/

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-with-a-dry-run on how to perform dry-runs.

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review