Improve explanation about option -N

Origin: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=766550#30
Bug-Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=345084
Bug-Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=530468

doc/w3m.1

@@ -77,7 +77,8 @@ visual startup mode
 monochrome display
 .TP
 .B -N
-open URL of command line on each new tab
+distribute multiple command line arguments to tabs. By default, a
+stack of buffers is used
 .TP
 .B -F
 automatically render frame