doc/nethack.6: Rewrite initial description

I noticed that the "Dungeons & Dragons" trademark acknowledgement should
be using the _registered_ trade mark sign instead of the more general
one, as the mark's enjoyed registered status (in the U.S.) since 1978.

https://tmsearch.uspto.gov/search/search-results/73123558

It occurred to me that updating the game description for a 2026 audience
might be an improvement on one that assumes the reader is familiar with
D&D, with Teletype machines ("TTYs"), and with the Rogue game, which
NetHack arguably eclipsed in notoriety decades ago.

So I rewrote it, and dropped the references to D&D.  Besides, as of
commit c99da87c70, NetHack is incompatible with "Strength Table I." in
the AD&D Player's Handbook (TSR 2010, p. 9), so any grognard who lifts
would snatch NetHack's D&D membership card away anyway.  ;-)

doc/nethack.6

@@ -123,10 +123,17 @@ can be any of the following.
 .hw nethackdir
 .SH DESCRIPTION
 .I NetHack
-is a display oriented Dungeons & Dragons(tm) - like game.
-The standard tty display and command structure resemble rogue.
+is a fantasy-themed game of surival,
+exploration,
+and combat
+set in the Mazes of Menace,
+an elaborate underground complex.
 .PP
-Other, more graphical display options exist for most platforms.
+Traditionally,
+the game's interface uses a Unix terminal connection,
+depicting the interface on a grid of character cells
+and responding to keyboard-entered commands.
+Other interface options exist for most platforms.
 .PP
 To get started you really only need to know two commands.
 The command
@@ -658,5 +665,3 @@ Probably infinite.
 This file is Copyright \*(co \*(Na, \*(Nd for version \*(Nb:\*(Nr.
 NetHack may be freely redistributed.
 See license for details.
-.PP
-Dungeons & Dragons is a Trademark of Wizards of the Coast, Inc.