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.

vncviewer/vncviewer.cxx

@@ -366,7 +366,8 @@ static void usage(const char *programName)
   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"

vncviewer/vncviewer.man

@@ -119,7 +119,11 @@ Specifies the X display on which the VNC viewer window should appear.
 .
 .TP
 .B \-geometry \fIgeometry\fP
-Standard X position and sizing specification.
+Initial position of the main VNC viewer window. The format is
+.B \fIwidth\fPx\fIheight\fP+\fIxoffset\fP+\fIyoffset\fP
+, where `+' signs can be replaced with `\-' signs to specify offsets from the
+right and/or from the bottom of the screen.  Offsets are optional and the
+window will be placed by the window manager by default.
 .
 .TP
 .B \-listen \fI[port]\fP