perf-security: document collected perf_events/Perf data categories

Document and categorize system and performance data into groups that
can be captured by perf_events/Perf and explicitly indicate the group
that can contain process sensitive data.

Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>