wallet_multisig_descriptor_psbt.py is refactored in this commit. While
behavior doesn't change we do cleanup the way wallets are accessed
throughout the test as this is done a lot for the various signers
and their multisigs. We also get rid of some shallow methods and
instead inline them for improved readability.
descriptors.md is improved to be more explicit about which wallet
(ie the signer or multisig) is required for each step.
This adds a new descriptor with syntax e.g. tr(KEY,{S1,{{S2,S3},S4})
where KEY is a key expression for the internal key and S_i are
script expression for the leaves. They have to be organized in
nested {A,B} groups, with exactly two elements.
tr() only exists at the top level, and inside the script expressions
only pk() scripts are allowed for now.
As described in "Key origin identification" section, a descriptor
that has hardened derivation after xpub does not let you compute scripts
without access to the corresponding private keys. Such a descriptor is
practically useless.
The text after the descriptor said "with child key *1'/2* of the
specified xpub", and clearly an xpub cannot have "child key" with
hardened derivation. Therefore it makes sense to fix this inconsistency
to not confuse the reader of the doc
