docs: staging/kprobes.rst: Move references to a separate appendix

Kprobes references are currently listed right after kretprobes example,
and appears to be part of the same section. Move this out to a separate
appendix for clarity.

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Link: https://lore.kernel.org/r/2bba61cd60eec87fbcca3e4343b7f95afa1178cd.1595331824.git.naveen.n.rao@linux.vnet.ibm.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>