[docs] Update tx-pool#transaction-priority doc

[sacha-l]

Update: WIP for the new devhub here.

This section needs an update based on what was introduced in this new PR in Substrate master.

[nuke-web3]

This should be addressed before the monthly-2021-11 release of substrate (and the devhub templates)

[lisa-parity]

I think we can close this given the doc in the new site.