hls: improve options description

doc/muxers.texi

@@ -106,10 +106,14 @@ avconv -i in.nut out.m3u8
 @end example
 
 @table @option
-@item -hls_time segment length in seconds
-@item -hls_list_size maximum number of playlist entries
-@item -hls_wrap number after which index wraps
-@item -start_number first number in the sequence
+@item -hls_time @var{seconds}
+Set the segment length in seconds.
+@item -hls_list_size @var{size}
+Set the maximum number of playlist entries.
+@item -hls_wrap @var{wrap}
+Set the number after which index wraps.
+@item -start_number @var{number}
+Start the sequence from @var{number}.
 @end table
 
 @anchor{image2}