perf evsel: Extend message to mention CAP_SYS_PTRACE and perf security doc link

perf evsel: Extend message to mention CAP_SYS_PTRACE and perf security doc link

Adjust limited access message to mention CAP_SYS_PTRACE capability for
processes of unprivileged users. Add link to perf security document in
the end of the section about capabilities.

The change has been inspired by this discussion:
https://lore.kernel.org/lkml/20200722113007.GI77866@kernel.org/

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>
Link: http://lore.kernel.org/lkml/6f8a7425-6e7d-19aa-1605-e59836b9e2a6@linux.intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>