Mention in the documentation that fieldmatch needs cfr input.

This can be improved in the decimate filter but for the moment
explicitely mentioning that mixed telecined and progressive
content is unsupported is an improvement.

Fixes ticket #3968.

doc/filters.texi

@@ -4447,6 +4447,10 @@ and VIVTC/VFM (VapourSynth project). The later is a light clone of TFM from
 which @code{fieldmatch} is based on. While the semantic and usage are very
 close, some behaviour and options names can differ.
 
+The filter currently only works for constant frame rate input. Do not use it
+if your input has mixed telecined and progressive content with changing
+framerate.
+
 The filter accepts the following options:
 
 @table @option