Update info on finalities

[gagdiez]

We are missing one finality parameter (near-final) which provides more guaranties than optimistic while being a block faster than final

[frol]

optimistic still requires a block to be produced and broadcasted, so in the worst case if transaction landed right after the previous block was produced it will take 1 block to wait until the next block will start forming, then 1 block to get included+converted to receipt in the block (at this point, 2 "seconds" since submission, the transaction will be in an "optimistic" block), then 1 block to execute the receipt + N blocks for cross-account interaction (if needed), and so if someone is looking for "optimistic transaction execution result", they will see it in the block T+3, where T is the optimistic block height at the moment of transaction submission, the block with the transaction result will become "near-final" at T+4 timestamp, and the block will become "final" at T+5 timestamp.

Also, with nearcore 2.6.0 (released yesterday to testnet), the block production time was reduced to 0.6s, so the seconds reference might need to be adjusted accordingly (T+3 will become 1.8s instead of 3s)