perf-security: document perf_events/Perf resource control

Extend perf-security.rst file with perf_events/Perf resource control
section describing RLIMIT_NOFILE and perf_event_mlock_kb settings for
performance monitoring user processes.

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