perf report: Document +field style argument support for --field option

perf report: Document +field style argument support for --field option

Commit ba9890b6ee13 ("perf tools: Add +field argument support for
--field option") by Jiri Olsa <jolsa@kernel.org> introduced +field style
argument support for --field option.

This is useful but not updated documentation.  This add a little
description there.

Signed-off-by: Changbin Du <changbin.du@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20170313083252.23644-1-changbin.du@intel.com
[ Slightly improved the phrase structure ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>