Clarify auto deploy schedule

What does this MR do?

Clarifies our auto-deploy schedule documentation to make it crystal clear slightly more clear when and how MRs need to be merged in order to be included in the monthly self-managed release.

Caveats

This MR is my best shot at clarifying our docs given my current understanding of our deploy process. It's very possible there are some gaps in my understanding. The goal of this MR is to get a discussion going.

Why?

During the last release (%12.10), there was a lot of confusion about what was and was not going to make it into the self-managed release on the 22nd, both within my team and others:

• gitlab-com/www-gitlab-com!41001 (comment 326248963)
• https://gitlab.slack.com/archives/CV09DAXEW/p1587146304450100?thread_ts=1587134801.443600&cid=CV09DAXEW (internal only)

The existing auto-deploy schedule documentation didn't contain quite enough information to answer the questions we had.