perf list: Document event specifications better

perf list: Document event specifications better

Document some features for specifying events in the perf list manpage:

- Event groups
- Leader sampling
- How to specify raw PMU events in the new syntax
- Global versus per process PMUs.
- Access restrictions
- Fix Intel SDM URL

v2: Lots of new content. address review feedback.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/1459810686-15913-1-git-send-email-andi@firstfloor.org
[ Add quotes to some keywords, such as "any" ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>