I was asked how a window-port controls which options are
set to SET_IN_FILE, DISP_IN_GAME, or SET_IN_GAME.
This provides a run-time way to change an option's SET_IN_FILE,
DISP_IN_GAME, or SET_IN_GAME status through code, rather
than clog up options.c with a lot of compile-time #ifdefs
for different ports to offer different default option settings.
Update the documentation to reflect this.
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