doc: Add release notes for 15629

Author: MarcoFalke

doc/release-notes.md

@@ -73,9 +73,14 @@ Example item
 Low-level changes
 =================
 
-Example item
+Configuration
 ------------
 
+* An error is issued where previously a warning was issued when a setting in
+  the config file was specified in the default section, but not overriden for
+  the selected network. This change takes only effect if the selected network
+  is not mainnet.
+
 Credits
 =======