Bolt 11: add distinct HRP prefix for Bitcoin signet (#844)

Judging from the comment
https://github.com/bitcoin/bitcoin/pull/18267/files#r491150895 in the
Signet PR all test networks should have the same bech32_hrp prefix (even
regtest). That's why 'tb' was chosen for Signet as well.
This is not optimal for LN as invoices shouldn't be vague in
what network they were issued for.
Therefore we add the explicit prefix 'lntbs' for Signet invoices.

11-payment-encoding.md

@@ -48,7 +48,8 @@ A reader:
 
 The human-readable part of a Lightning invoice consists of two sections:
 1. `prefix`: `ln` + BIP-0173 currency prefix (e.g. `lnbc` for Bitcoin mainnet,
-   `lntb` for Bitcoin testnet, and `lnbcrt` for Bitcoin regtest)
+   `lntb` for Bitcoin testnet, `lntbs` for Bitcoin signet, and `lnbcrt` for
+   Bitcoin regtest)
 1. `amount`: optional number in that currency, followed by an optional
    `multiplier` letter. The unit encoded here is the 'social' convention of a payment unit -- in the case of Bitcoin the unit is 'bitcoin' NOT satoshis.