feh(1): Add note that the scroll keys don't antialias

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.It Ao keypad down Ac , Ao Ctrl+Down Ac Bq scroll_down

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Scroll down

Scroll down.

Note that the scroll keys work without anti-aliasing for performance reasons,

hit the render key after scrolling to antialias the image.

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.It Ao keypad begin Ac Bq render

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