[FLINK-22244][docs] Adjust Reactive Mode docs

[rmetzger]

What is the purpose of the change

As part of the release testing, there was some feedback about the Adaptive Scheduler timeouts not being defined properly. I linked the respective configuration keys in the docs and clarified them a bit.

I also made some additional adjustments to the docs.

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[tillrohrmann]

Thanks for creating this PR @rmetzger. LGTM. +1 for merging.

[rmetzger]

Thanks a lot for your review. I'll merge this PR soon!