Minor documentation fixup

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 Aq Alt+Left

.It Aq Alt+Left Bq scroll_left_page

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Scroll to the left by one page

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.It Aq Alt+Right

.It Aq Alt+Right Bq scroll_right_page

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Scroll to the right by one page

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.It Aq Alt+Up

.It Aq Alt+Up Bq scroll_up_page

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Scroll up by one page

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.It Aq Alt+Down

.It Aq Alt+Down Bq scroll_down-page

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Scroll down by one page

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 -w, --multiwindow         Open all files at once, one window per image

 -x, --borderless          Create borderless windows

 -d, --draw-filename       Show the filename in the image window

     --draw-tinted         Show overlay texts on semi-transparent background

 -^, --title TITLE         Set window title (see FORMAT SPECIFIERS)

 -D, --slideshow-delay NUM Set delay between automatically changing slides

     --cycle-once          Exit after one loop through the slideshow