BACKPORT: perf tools: Document the perf sysctls

perf_event_paranoid was only documented in source code and a perf error
message.  Copy the documentation from the error message to
Documentation/sysctl/kernel.txt.

BACKPORT notes:
The error printing from upstream does not exist in the 3.4 kernel.
Only backporting the documentation update from this commit.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-doc@vger.kernel.org
Link: http://lkml.kernel.org/r/20160119213515.GG2637@decadent.org.uk


[ Remove reference to external Documentation file, provide info inline, as before ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

Bug: 29054680
Change-Id: I13e73cfb2ad761c94762d0c8196df7725abdf5c5