Identify known limitations that have been resolved

[jseldess]

In addition to documenting new 20.1 known limitations, we need to review the "unresolved" limitations, figure out if any have been resolved in the meantime, and update our 20.1 doc as appropriate.

[piyush-singh]

All limitations mentioned for the Admin UI are still valid. Did a full run through and to the best of my knowledge the limitations for Observability and Security in general are still valid.

[jseldess]

Thanks, @piyush-singh!

[johnrk-zz]

From @andreimatei :
"Write and update limits for a single statement - I’d probably get rid of this one. There hasn’t been a limit per statement for a long time. The limit is now per… I think groups of 10k rows being written by the statement.
Cold starts of large clusters may require manual intervention - This one probably needs to be adjusted based on the new default range size. Also I think we should probably nudge people with existing clusters to up their range size limits to match the defaults."

[jseldess]

Thanks, @johnrk. I'll hand the former. @rmloveland, can you handle the latter update related to range size? I think you have a PR open on the topic.