doc/nethack.6: Revise role info in synopsis

Revise presentation of backward-compatible role options.

- Give them a metasyntactic variable name and present them in the main
  command synopsis.

- Present the possible values in a typographical display, indented with
  filling disabled.

doc/nethack.6

@@ -55,6 +55,7 @@ nethack \- Exploring The Mazes of Menace
 .nr sA \n(.j \" Save the adjustment mode.
 .na
 .B nethack
+.RI [ role-option ]
 .RB [ \-d | \-\-directory\*_\c
 .IR directory ]
 .RB [ \-w | \-\-windowtype\*_\c
@@ -75,34 +76,26 @@ nethack \- Exploring The Mazes of Menace
 .IR race ]
 .RB [ \-@ ]
 .PP
-Also
-[
+.I role-option
+can be any of the following.
+.PP
+.RS
+.nf
 .BR \-A | \-Arc
-|
 .BR \-B | \-Bar
-|
 .BR \-C | \-Cav
-|
 .BR \-H | \-Hea
-|
 .BR \-K | \-Kni
-|
 .BR \-M | \-Mon
-|
 .BR \-P | \-Pri
-|
 .BR \-R | \-Rog
-|
 .B \-Ran
-|
 .BR \-S | \-Sam
-|
 .BR \-T | \-Tou
-|
 .BR \-V | \-Val
-|
 .BR \-W | \-Wiz
-]
+.fi
+.RE
 .HP
 .B nethack
 .RB { \-s | \-\-scores }