perf: Document the new transaction sample type

Commit fdfbbd07e91f8fe3871 ("perf: Add generic transaction flags")
added support for PERF_SAMPLE_TRANSACTION but forgot to add documentation
for the sample type to include/uapi/linux/perf_event.h

Signed-off-by: Vince Weaver <vincent.weaver@maine.edu>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: Andi Kleen <ak@linux.intel.com>
Link: http://lkml.kernel.org/r/alpine.DEB.2.02.1312131548450.10372@pianoman.cluster.toy
Signed-off-by: Ingo Molnar <mingo@kernel.org>