Update docs related to transaction build process

[TalDerei]

References #3401

transaction_signing.md doc is modified to describe the build process in more detail according to the changes in #3289. Additionally, since the UnauthTransaction type was removed, I propose changing the intermediate state of a transaction to Unauthenticated Transaction or Pending Transaction for improved clarity.

[hdevalence]

Additionally, since the UnauthTransaction type was removed, I propose changing the intermediate state of a transaction to UnauthenticatedTransaction or PendingTransaction for improved clarity.

One thing that's a little awkward is that now we don't actually have a different state, it's just a transaction (with invalid data in it). Not sure the best way to handle this in the docs.

[hdevalence]

Here we should probably say "unauthenticated Transaction", right, since there's no distinct data type / message?

[TalDerei]

good point, changed

[TalDerei]

Additionally, since the UnauthTransaction type was removed, I propose changing the intermediate state of a transaction to UnauthenticatedTransaction or PendingTransaction for improved clarity.

One thing that's a little awkward is that now we don't actually have a different state, it's just a transaction (with invalid data in it). Not sure the best way to handle this in the docs.

It might be better to describe it as a state transition in terms of the Transaction internals changing across the discrete build phases. I think we kind of already do that with our current description: "This intermediate state of the transaction without the full authorizing data..."?