linux-phytec-ti

perf trace: update Documentation with new perf trace variants

Add documentation describing new 'perf trace' command changes
e.g. <command> handling and live-mode/top variants.

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Acked-by: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>