doc/filters: note requirements for xfade inputs

GyanD · GyanD · commit 54d8386f5254 · 2021-04-20T09:32:50.000+05:30
diff --git 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.
 
+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
