Documentation: kprobes: Document jprobes stack copying limitations

Some architectures (i.e.: sparc64 and arm64) make reasonable partial stack
duplication for jprobes problematic. Document this.

Signed-off-by: David A. Long <dave.long@linaro.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>