tracing/kprobes: Add short documentation for HAVE_REGS_AND_STACK_ACCESS_API

So that arch developers know how to implement it without the
need to dig into changelogs.

Reported-by: Mike Frysinger <vapier.adi@gmail.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-by: Masami Hiramatsu <mhiramat@redhat.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: "David S . Miller" <davem@davemloft.net>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <20100218132521.GB2406@osiris.boeblingen.de.ibm.com>
[added reference to ptrace.h in the config help]
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>