Expand on explanation for multiprocess-mode in roundup-server.1.

Author: rouilj

share/man/man1/roundup-server.1

@@ -21,7 +21,9 @@ file indicated by PIDfile. The -l (or -L) option \fBmust\fP be
 specified if -d is used.
 .TP
 \fB-t\fP \fBfork|thread|debug|none\fP
-Control multi-process mode. Default fork.
+Control multi-process mode. \fBdebug\fP and \fBnone\fP are always
+available. If an invalid mode is specifie the server starts in
+\fBnone\fP (also called single) mode. Default fork.
 .TP
 \fB-l\fP \fIfile\fP
 Sets a filename to log to (instead of stdout). This is required if the -d