doc/filters: note requirements for xfade inputs

diff --git a/doc/filters.texi b/doc/filters.texi
index 68f17dd5630f43f0a6c96a48ccae1a1701d4c855..67587d96ee7dfcc0c26c7a26a02f7d75656bbfdc 100644 (file)
--- a/doc/filters.texi
+++ b/doc/filters.texi
@@ -21803,6 +21803,9 @@ Default is @code{3}.
 Apply cross fade from one input video stream to another input video stream.
 The cross fade is applied for specified duration.
 
 Apply cross fade from one input video stream to another input video stream.
 The cross fade is applied for specified duration.
 

+Both inputs must be constant frame-rate and have the same resolution, pixel format,
+frame rate and timebase.
+

 The filter accepts the following options:
 
 @table @option
 The filter accepts the following options:
 
 @table @option