ffmpeg: document the unit for the -fs option parameter

doc/ffmpeg.texi

@@ -143,7 +143,7 @@ Stop writing the output after its duration reaches @var{duration}.
 @var{duration} may be a number in seconds, or in @code{hh:mm:ss[.xxx]} form.
 
 @item -fs @var{limit_size} (@emph{output})
-Set the file size limit.
+Set the file size limit, expressed in bytes.
 
 @item -ss @var{position} (@emph{input/output})
 When used as an input option (before @code{-i}), seeks in this input file to

ffmpeg.c

@@ -305,7 +305,7 @@ typedef struct OutputFile {
     int ost_index;       /* index of the first stream in output_streams */
     int64_t recording_time; /* desired length of the resulting file in microseconds */
     int64_t start_time;     /* start time in microseconds */
-    uint64_t limit_filesize;
+    uint64_t limit_filesize; /* filesize limit expressed in bytes */
 } OutputFile;
 
 static InputStream *input_streams   = NULL;