Better document the argument for -geometry

It's not common knowledge these days how X geometry is specified, and
even the man page X(7) is missing on many systems now.
diff --git a/vncviewer/vncviewer.cxx b/vncviewer/vncviewer.cxx
index f9e58b9..d7cbd6e 100644
--- a/vncviewer/vncviewer.cxx
+++ b/vncviewer/vncviewer.cxx
@@ -366,7 +366,8 @@
   fprintf(stderr,"\n"
           "Options:\n\n"
           "  -display Xdisplay  - Specifies the X display for the viewer window\n"
-          "  -geometry geometry - Standard X position and sizing specification.\n");
+          "  -geometry geometry - Initial position of the main VNC viewer window. See the\n"
+          "                       man page for details.\n");
 #endif
 
   fprintf(stderr,"\n"