--- a/doc/hgrc.5.txt
+++ b/doc/hgrc.5.txt
@@ -257,7 +257,9 @@ ui::
   username;;
     The committer of a changeset created when running "commit".
     Typically a person's name and email address, e.g. "Fred Widget
-    <fred@example.com>".  Default is $EMAIL or username@hostname.
+    <fred@example.com>".  Default is $EMAIL or username@hostname, unless
+    username is set to an empty string, which enforces specifying the
+    username manually.
   verbose;;
     Increase the amount of output printed.  True or False.  Default is False.