commands: describe --debug in showconfig help

mercurial/commands.py

@@ -784,7 +784,11 b' def showconfig(ui, repo, *values, **opts'
     that config item.
 
     With multiple args, print names and values of all config items
-    with matching section names."""
+    with matching section names.
+
+    With the --debug flag, the source (filename and line number) is
+    printed for each config item.
+    """
 
     untrusted = bool(opts.get('untrusted'))
     if values: