Expand zero-confirmation payment guidance

[KingParmenides]

Summary

• Expands the payment verification section's zero-confirmation guidance.
• Explains mempool non-consensus, unconfirmed double-spend risk, and how that differs from chain reorg double spends.
• Adds guidance on low-fee/stuck transactions, RBF, CPFP, and locktime constraints.

Bounty

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

BTC payout address: 39Q34P8A7g375yqEr8buvNJkUbRgKfbKQZ

Testing

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

Build succeeded. Existing warnings remain for conf.py language=None and glossary.rst:647; no new warnings from this change.