transactions.md: generalize the unvault policy to allow non-stakeholder fund managers

[darosior]

This implicitly allows:

• Non-stakeholders fund managers.
• Threshold conditions for fund managers.

Coming at the cost of optimisation:

• This requires different keys to be used by fund managers if they are part of the stakeholders.
• Some cosigner pubkey requirements might be redundant.

This fixes https://github.com/re-vault/practical-revault/issues/19 .

Unfortunately, i could not establish accurate measurements (i tried) of the new limitations in term of number of each member of each set due to a current bug in rust-miniscript.

So, the updated pretty graphic will come soon :tm: !

The implementation is available here.

[darosior]

For posterity: Kevin acked out of band