[FLINK-22253][docs] Update back pressure monitoring docs with new WebUI changes

[pnowojski]

This PR updates the docs to reflect changes implemented in https://issues.apache.org/jira/browse/FLINK-14712

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Last check on commit 8cd8179 (Fri May 28 11:07:56 UTC 2021)

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[alpinegizmo]

The explanations are clear, I simply created some suggestions for improving the English.

One question though: does the timeout work as stated? I remember we discussed with Roman doing this a bit differently.

[pnowojski]

Thanks for the suggestions @alpinegizmo! Yes, uc timeout is counted against subtask's alignment time.

[alpinegizmo]

I'll approve this now, but I've added one last (trivial) suggestion.

[pnowojski]

Thanks a lot! I've included that last one suggestion and I'm going to merge it soon.