transaction-pool: Improve transaction status documentation and add helpers (#3215)

This PR improves the transaction status documentation.
- Added doc references for describing the main states
- Extra comment wrt pool ready / future queues
- `FinalityTimeout` no longer describes a lagging finality gadget, it
signals that the maximum number of finality gadgets has been reached

A few helper methods are added to indicate when:
- a final event is generated by the transaction pool for a given event
- a final event is provided, although the transaction might become valid
at a later time and could be re-submitted

The helper methods are used and taken from
https://github.com/paritytech/polkadot-sdk/pull/3079 to help us better
keep it in sync.


cc @paritytech/subxt-team

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Signed-off-by: Alexandru Vasile <[email protected]>