doc/avconv: extend -ss documentation.

Signed-off-by: NAlex Converse <alex.converse@gmail.com>

doc/avconv.texi

@@ -134,8 +134,12 @@ to the duration specified in seconds.
 Set the file size limit.
 
 @item -ss @var{position}
-Seek to given time position in seconds.
-@code{hh:mm:ss[.xxx]} syntax is also supported.
+When used as an input option (before @code{-i}), seeks in this input file to
+@var{position}. When used as an output option (before an output filename),
+decodes but discards input until the timestamps reach @var{position}. This is
+slower, but more accurate.
+
+@var{position} may be either in seconds or in @code{hh:mm:ss[.xxx]} form.
 
 @item -itsoffset @var{offset}
 Set the input time offset in seconds.