Document the signing behavior from version 2.4.0

[bigspider]

Since version 2.4.0, the app has a fine-grained signing behavior, allowing to avoid signing unnecessarily for all possible spending paths if only some are present. This should be better documented.

For MuSig2, the app's decisions are affected by the presence of pubnonces, and this might not match the expectations of people working with the PSBT signing flow of musig (particularly, the app assumes Round 2 if any pubnonce is present in the PSBT.

TBD if we also want to modify the client libraries to handle this more gracefully (e.g.: if some pubnonces are present, but not all - then strip all of them, so the app proceeds to Round 1 instead of returning an error later).