doc/admin-guide: Note credentials consolidation under CAP_PERFMON

Add note that starting from Linux v5.9 CAP_PERFMON Linux capability is
enough to conduct performance monitoring and observability using
perf_events API.

Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-doc@vger.kernel.org
Cc: linux-man@vger.kernel.org
Cc: linux-security-module@vger.kernel.org
Link: http://lore.kernel.org/lkml/2b1a92a1-84ce-5c70-837d-8ffe96849588@linux.intel.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>