ZIP 244: Clarify that shielded inputs can't sign the txid if there is a transparent bundle

[str4d]

The Signature Digest first paragraph states:

A new per-input transaction digest algorithm is defined that constructs a hash that may be signed by a transaction creator to commit to the effects of the transaction. A signature digest is produced for each transparent input, each Sapling input, and each Orchard action. For transparent inputs, this follows closely the algorithms from ZIP 143 and ZIP 243. For shielded inputs, this algorithm has the exact same output as the transaction digest algorithm, thus the txid may be signed directly.

This last sentence was an original design goal, and IIRC was originally correct, but due to changes later on in the design of ZIP 244, it is now incorrect because the sighash commits to information about the transparent outputs that are being spent (such as their amounts) in order to make the lives of hardware wallets easier.

Fix this paragraph to say something like:

This follows closely the algorithms from ZIP 143 and ZIP 243. For transactions without transparent inputs, this algorithm has the exact same output as the transaction digest algorithm, thus the txid may be signed directly.