doc/filters: prefer "special" over "magic" for special input/output labels

doc/filters.texi

@@ -39,7 +39,7 @@ chain are separated by commas. In our example, @var{split, fifo,
 overlay} are in one linear chain, and @var{fifo, crop, vflip} are in
 another. The points where the linear chains join are labeled by names
 enclosed in square brackets. In our example, that is @var{[T1]} and
-@var{[T2]}. The magic labels @var{[in]} and @var{[out]} are the points
+@var{[T2]}. The special labels @var{[in]} and @var{[out]} are the points
 where video is input and output.
 
 Some filters take in input a list of parameters: they are specified