vncserver: document bypassing DE selection prompt

unix/vncserver.man

@@ -54,11 +54,18 @@ passed to Xvnc - see the Xvnc man page, or "Xvnc \-help", for details.
 
 .TP
 .B \-select-de [\fIde_name\fP]
-Select Desktop Enviromnent to run. Cinnamon, Mate, LXDE, LXQT, KDE, Gnome, XFCE
+Select Desktop Enviromnent to run. Cinnamon, Mate, LXDE, LXQT, KDE, Gnome, XFCE,
-are supported. If \fIde_name\fP isn't specified, a text UI prompt to select a
+are supported.
-Desktop Enviromnent will be shown.
+
+To trigger a text UI prompt to select a Desktop Enviromnent, run \fBvncserver\fP
+for the first time, or pass this option without arguments. To bypass the prompt,
+pass this option with the Desktop Enviromnent argument.
 Warning: $HOME/.vnc/xstartup will be overwritten.
 
+To run a custom setup (not one of the supported Desktop Enviromnents), create
+$HOME/.vnc/xstartup executable and specify \fB\-select-de manual\fP to signify you
+don't want its contents to be auto-generated.
+
 .TP
 .B \-geometry \fIwidth\fPx\fIheight\fP
 Specify the size of the VNC desktop to be created. Default is 1024x768.