--- a/doc/Makefile
+++ b/doc/Makefile
@@ -65,6 +65,10 @@
+# The images included in the HTML manual must be present before the makeinfo
+# command is invoked or it will fall back on incorrect file names.
 # The texi2dvi script (used to create both PDF and DVI output formats)
 # uses some fixed temporary file names.  In order to avoid a race condition
 # the DVI and PDF builds are forced to run serially through a Makefile rule.