doc: add signet to doc/bitcoin-conf.md

Author: jonatack

doc/bitcoin-conf.md

@@ -27,7 +27,7 @@ Comments may appear in two ways:
 ### Network specific options
 
 Network specific options can be:
-- placed into sections with headers `[main]` (not `[mainnet]`), `[test]` (not `[testnet]`) or `[regtest]`;
+- placed into sections with headers `[main]` (not `[mainnet]`), `[test]` (not `[testnet]`), `[signet]` or `[regtest]`;
 - prefixed with a chain name; e.g., `regtest.maxmempool=100`.
 
 Network specific options take precedence over non-network specific options.