Document Environment Auto Stop on MR merge/close events

What does this MR do and why?

There is undocumented behavior on Environment Auto-Stop that, an environment can be stopped at MR merge/close events. This confuses the users in #299412 (closed).

This MR documents the current behavior.

Related #299412 (closed)

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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