doc/avconv: expand documentation for the -s option.

doc/avconv.texi

@@ -260,8 +260,18 @@ Set the number of video frames to record. This is an alias for @code{-frames:v}.
 Set frame rate (Hz value, fraction or abbreviation), (default = 25). For output
 streams implies @code{-vsync cfr}.
 @item -s[:@var{stream_specifier}] @var{size} (@emph{input/output,per-stream})
-Set frame size. The format is @samp{wxh} (default - same as source).
-The following abbreviations are recognized:
+Set frame size.
+
+As an input option, this is a shortcut for the @option{video_size} private
+option, recognized by some demuxers for which the frame size is either not
+stored in the file or is configurable -- e.g. raw video or video grabbers.
+
+As an output option, this inserts the @code{scale} video filter to the
+@emph{end} of the corresponding filtergraph. Please use the @code{scale} filter
+directly to insert it at the beginning or some other place.
+
+The format is @samp{wxh} (default - same as source).  The following
+abbreviations are recognized:
 @table @samp
 @item sqcif
 128x96