Expand unconfirmed payment risk guidance

[0xjc65eth]

Fixes bitcoin-dot-org/Bitcoin.org#1216.

Summary

• Expands the 0-confirmation payment guidance to use precise "unconfirmed transaction" terminology.
• Explains why mempool visibility is local relay-policy evidence rather than consensus.
• Adds practical risk signals for unconfirmed payment acceptance: peer propagation, conflicts, fee rate, value limits, and manual review.
• Covers stuck low-fee payments, RBF/CPFP, CLTV/CSV limits, and refund/support workflows.
• Distinguishes unconfirmed on-chain payments from off-chain protocols such as Lightning Network payments.

Bounty

This PR targets the 25,000 bits / ~$250 USD bounty in bitcoin-dot-org/Bitcoin.org#1216.

BTC payout address: bc1qjf2t6dc75c6t2948essyrqec0a08l8zl28fcfh

Verification

• git diff --check
• PATH=/tmp/developer-bitcoin-venv/bin:$PATH make clean html

The Sphinx build succeeds. The only warnings are the repository's existing warnings for conf.py language = None and glossary.rst:647; this change adds no new warnings in devguide/payment_processing.rst.