Add config changes to release notes

doc/release-notes-pr12823.md

@@ -0,0 +1,20 @@
+Configuration sections for testnet and regtest
+----------------------------------------------
+
+It is now possible for a single configuration file to set different
+options for different networks. This is done by using sections or by
+prefixing the option with the network, such as:
+
+    main.uacomment=bitcoin
+    test.uacomment=bitcoin-testnet
+    regtest.uacomment=regtest
+    [main]
+    mempoolsize=300
+    [test]
+    mempoolsize=100
+    [regtest]
+    mempoolsize=20
+
+The `addnode=`, `connect=`, `port=`, `bind=`, `rpcport=`, `rpcbind=`
+and `wallet=` options will only apply to mainnet when specified in the
+configuration file, unless a network is specified.