Clean up and document nuclear damage's chance to destroy nuke

Change chance in percent from damage-1 to damage.  It's always been
damage-1, but I can't bring myself to document the silly -1.

info/Damage.t

@@ -87,6 +87,10 @@ Land unit
 Plane
 %damage = damage
 
+Nuke
+can't be damaged by conventional weapons
+nuclear damage may destroy it (chance in percent equal to damage)
+
 Sector
 sectdamage = damage / ((sectdef - 2) * eff + 2)
 %damage = (100 * sectdamage / (sectdamage + 100))
@@ -97,5 +101,8 @@ planes take 1/7 of the damage done to land units
 Commodity
 %damage = (100 * damage / (damage + 50))
 
+Note: when a ship, plane, land unit or bridge is destroyed, its
+contents is lost.  This includes nukes.
+
 .fi
 .SA "fire, launch, torpedo, lmine, bomb, Hitchance, Attacking, Combat"

src/lib/subs/detonate.c

@@ -28,7 +28,7 @@
  *
  *  Known contributors to this file:
  *     Steve McClure, 1998-2000
- *     Markus Armbruster, 2004-2010
+ *     Markus Armbruster, 2004-2012
  */
 
 #include <config.h>
@@ -279,7 +279,7 @@ detonate(struct nukstr *np, coord x, coord y, int airburst)
 	    continue;
 	if ((damage = nukedamage(ncp, ni.curdist, airburst)) <= 0)
 	    continue;
-	if (roll(100) >= damage)
+	if (roll(100) > damage)
 	    continue;
 	nuke.nuk_effic = 0;
 	if (own == player->cnum) {