doc/ffmpeg - update benchmark option readout details

-benchmark and -benchmark_all now show real, system and user time

doc/ffmpeg.texi

@@ -1154,12 +1154,12 @@ disable any chapter copying.
 
 @item -benchmark (@emph{global})
 Show benchmarking information at the end of an encode.
-Shows CPU time used and maximum memory consumption.
+Shows real, system and user time used and maximum memory consumption.
 Maximum memory consumption is not supported on all systems,
 it will usually display as 0 if not supported.
 @item -benchmark_all (@emph{global})
 Show benchmarking information during the encode.
-Shows CPU time used in various steps (audio/video encode/decode).
+Shows real, system and user time used in various steps (audio/video encode/decode).
 @item -timelimit @var{duration} (@emph{global})
 Exit after ffmpeg has been running for @var{duration} seconds.
 @item -dump (@emph{global})