BIP 370: add missing requirement for PSBT_OUT_AMOUNT field

Author: darosior

bip-0370.mediawiki

@@ -189,7 +189,7 @@ The new per-output types for PSBT Version 2 are defined as follows:
 | None
 | No key data
 | <tt><64-bit little endian int amount></tt>
-| 64 bit signed little endian integer representing the output's amount in satoshis.
+| 64 bit signed little endian integer representing the output's amount in satoshis. Must be omitted in PSBTv0. Must be provided in PSBTv2.
 | 2
 | 0
 | 2