Improve documentation for the -pass and -passlog options. Add mention

to how the log file is used and specify the default log file name.

Originally committed as revision 15969 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/ffmpeg-doc.texi

@@ -439,10 +439,11 @@ tell that the raw codec data must be copied as is.
 Use same video quality as source (implies VBR).
 
 @item -pass @var{n}
-Select the pass number (1 or 2). It is useful to do two pass
-encoding. The statistics of the video are recorded in the first
-pass and the video is generated at the exact requested bitrate
-in the second pass.
+Select the pass number (1 or 2). It is used to do two-pass
+video encoding. The statistics of the video are recorded in the first
+pass into a log file (see also the option -passlogfile),
+and in the second pass that log file is used to generate the video
+at the exact requested bitrate.
 On pass 1, you may just deactivate audio and set output to null,
 examples for Windows and Unix:
 @example
@@ -451,7 +452,9 @@ ffmpeg -i foo.mov -vcodec libxvid -pass 1 -an -f rawvideo -y /dev/null
 @end example
 
 @item -passlogfile @var{file}
-Set two pass logfile name to @var{file}.
+Set two-pass log file name to @var{file}. Default name is
+@file{ffmpeg2pass-N.log}, where N is a number specific to the output
+stream.
 
 @item -newvideo
 Add a new video stream to the current output stream.