Slack app review blocker: Make sure copy for GitLab for Slack app events is accurate

This issue came out of #384226 (comment 1251645250).

Problem

Some of the trigger descriptions on the GitLab for Slack app integration page is inaccurate.

For example, we say:

An issue is created, updated, or closed

When the trigger does not seem to happen with every update, but rather appears to only trigger when an issue is created, closed, or reopened.

Proposal

1. Review the copy on the GitLab for Slack app integration page and make sure the description of the triggers aligns with what actually happens (look for the calls to #execute_integrations in things like services, or models, to determine when they trigger)
2. Make changes to the copy on that page with help by a Technical writer.