Approvals page: explain self-approval, cut wordiness

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What does this MR do?

In #464648 (closed), I need to document that we've added an extra cue so users can tell at a glance if they've personally approved a merge request:

There wasn't a really good place in https://docs.gitlab.com/ee/user/project/merge_requests/approvals/ for this. I started scrubbing text a bit, looking for ways to reshape the text. This MR definitely isn't done yet, but I'm unsure if I've hit MVC or not. I'll send it over to Marcin, take advantage of time zones, and get his thoughts. Iterate? Merge this first part? I think it's a little half-baked but he might disagree!

(The way I know it's not 100% done: I need to explain the various icons shown in the right sidebar of merge requests, and I've tackled only the merge request list.)

I've kicked off a review app.

Related issues

Related to Document icon usage in Code Review (#464648 - closed)

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