GB: Shadow sprite revives faint lines

goombobros reported in gbdev Discord server that a scratchy power
switch can burn in rows of the LCD, and setting the contrast dial
to black and rapidly turning LCD on and off in software can
revive it.  Guess what Shadow sprite does while changing BGs.

Thank you goombobros.

Author: pinobatch

gameboy/src/helppages.txt

@@ -289,7 +289,19 @@ Portrait of Donna by
   pepe_salot
 Hill zone by mikejmoffitt
   & Damian Yerrick
-
+----
+Got burn-in?
+
+A scratchy power switch
+on a monochrome system
+can cause horizontal
+rows of dots to fade.
+goombobros reported that
+turning the contrast dial
+to black and changing
+background (A+→) for
+a couple minutes can
+revive faint lines.
 == Stopwatch ==
 
 This stopwatch helps