clarify bip68 and bip112 description of required transaction versions

[instagibbs]

https://delvingbitcoin.org/t/disclosure-btcd-consensus-bugs-due-to-usage-of-signed-transaction-version/455

Seems like a good idea to be clear up front on the nversion requirements even if we're relying on the implementation-as-spec aspects of the bips.

[Roasbeef]

@NicolasDorier

[murchandamus]

@instagibbs, this PR has had unaddressed review comments for three months. Are you still working on this?

[instagibbs]

I'm not planning on changing anything further unless people feel strongly about @vostrnad 's suggestions(which seem fine to me as well)

[Christewart]

FWIW, i prefer @instagibbs 's current wording.

[vostrnad]

To elaborate: as is, the BIP implicitly treats the version as an unsigned integer. This PR switches to (still implicitly) treating it as a signed integer. My proposed changes would explicitly treat it as an unsigned integer. Given that Bitcoin Core will likely switch from interpreting the version as signed to unsigned (https://github.com/bitcoin/bitcoin/pull/29325), having it unsigned here as well seems like the best way forward (and being explicit about it is a strict improvement in any case).

[jonatack]

@NicolasDorier mind weighing in here?

[instagibbs]

closing, I think @vostrnad has right way forward, anyone can grab it