Index: doc/options.txt
===================================================================
--- doc/options.txt	(revision 4642)
+++ doc/options.txt	(working copy)
@@ -617,6 +617,10 @@
 
 === Diagnostic options ===
 
+Messages produced by the diagnostic options are directed to stderr when no logging
+configuration file is in use. When using a logging configuration file, they are logged
+with custom level DIAGNOSTIC (1100).
+
 ;--check-roundabouts
 : 	Check that roundabouts have the expected direction (clockwise
 when vehicles drive on the left). Roundabouts that are complete