doc/filters: extend midequalizer description

Signed-off-by: Paul B Mahol <onemda@gmail.com>

doc/filters.texi

@@ -9647,6 +9647,14 @@ Search parameter. Default @code{7}.
 
 Apply Midway Image Equalization effect using two video streams.
 
+Midway Image Equalization adjusts a pair of images to have the same
+histogram, while maintaining their dynamics as much as possible. It's
+useful for e.g. matching exposures from a pair of stereo cameras.
+
+This filter has two inputs and one output, which must be of same pixel format, but
+may be of different sizes. The output of filter is first input adjusted with
+midway histogram of both inputs.
+
 This filter accepts the following option:
 
 @table @option