Follow-up to #21544 and #20891 for the bumpfee_helper
used for RPCs bumpfee and psbtbumpfee:
- “psbt” field is only returned in psbtbumpfee and not bumpfee
- bumpfee raises if private keys are disabled, so the txid help “Only returned when wallet private keys are enabled.” no longer makes sense; remove it
- add missing space in RPC examples (“Bump the fee, get the new transaction’stxid”)
- update txid/psbt code comments