doc/filters: rework introductory description for decimate

In particular, dedicate first paragraph to deliver a short description of
the filter.

doc/filters.texi

@@ -2063,11 +2063,12 @@ playback.
 
 @section decimate
 
-This filter drops frames that do not differ greatly from the previous
-frame in order to reduce framerate.  The main use of this filter is
-for very-low-bitrate encoding (e.g. streaming over dialup modem), but
-it could in theory be used for fixing movies that were
-inverse-telecined incorrectly.
+Drop frames that do not differ greatly from the previous frame in
+order to reduce framerate.
+
+The main use of this filter is for very-low-bitrate encoding
+(e.g. streaming over dialup modem), but it could in theory be used for
+fixing movies that were inverse-telecined incorrectly.
 
 The filter accepts parameters as a list of @var{key}=@var{value}
 pairs, separated by ":". If the key of the first options is omitted,