Update --geometry documentation in feh(1)

Takes an X-style geometry

.Ar string

like 640x480 with optional +x+y window offset.

Note that larger images will be zoomed out to fit, but you can see them at 1:1

by clicking the zoom button.

Use

.Cm --scale-down

to scale down larger images like in fullscreen mode.

.

Also note that this option does not enforce the geometry, changing it by a tiling

WM or manually is still possible.