perf symbols: Add description of JIT interface

Add a description of the JIT interface in the perf symbol resolution
code. I reverse engineered the format from the source.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>
Link: http://lkml.kernel.org/r/1344526260-18721-1-git-send-email-andi@firstfloor.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>