doc/admin-guide: Document creation of CAP_PERFMON privileged shell

Document steps to create CAP_PERFMON privileged shell to unblock Perf
tool usage in cases when capabilities can't be assigned to an executable
due to limitations of used file system.

Suggested-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
Cc: Alexander Shishkin <alexander.shishkin@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/0abda956-de6c-95b1-61e8-49e146501079@linux.intel.com


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