feh(1): move --xinerama-index to background setting section, extend description

Disable Xinerama support.  Only makes sense when you have Xinerama support

compiled in.

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.It Cm --xinerama-index

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Force Xinerama screen index to use.  Only makes sense when you have 

Xinerama support compiled in and using

.Nm

as a background setter.

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.It Cm -j , --output-dir Ar directory

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Save files to

.Pa ~/.fehbg

file

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.It Cm --xinerama-index Ar screen

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When used with any option other than

.Cm --bg-tile :

Only set wallpaper on

.Ar screen .

All other screens will be filled black/white.

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This is most useful in a Xinerama configuration with overlapping screens.

For instance, assume you have two overlapping displays (index 0 and 1), where

index 0 is smaller. To center a background on the display with index 0 and

fill the extra space on index 1 black/white, use

.Qq --xinerama-index 0

when setting the wallpaper.

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.El

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