BIP-0322: clarify scriptSig on to_sign for legacy transactions

Before this commit it was not clear that non-native SegWit outputs
(e.g. P2PKH or P2SH-P2WPKH) only work if the correct scriptSig is
provided.
This then also makes it more clear why P2SH-P2WPKH outputs are NOT
supported by the "simple" variant.

Author: guggero

bip-0322.mediawiki

@@ -91,6 +91,7 @@ The <code>to_sign</code> transaction is:
     vin[0].prevout.hash = to_spend.txid
     vin[0].prevout.n = 0
     vin[0].nSequence = 0 or (FULL format only) as appropriate (for time locks)
+    vin[0].scriptSig = [] or (FULL format only) as appropriate (for non segwit-native transactions)
     vin[0].scriptWitness = message_signature
     vout[0].nValue = 0
     vout[0].scriptPubKey = OP_RETURN