docs: move remaining stuff under Documentation/*.txt to Documentation/staging

There are several files that I was unable to find a proper place
for them, and 3 ones that are still in plain old text format.

Let's place those stuff behind the carpet, as we'd like to keep the
root directory clean.

We can later discuss and move those into better places.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/11bd0d75e65a874f7c276a0aeab0fe13f3376f5f.1592203650.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

samples/kprobes/kprobe_example.c

@@ -5,7 +5,7 @@
  * stack trace and selected registers when _do_fork() is called.
  *
  * For more information on theory of operation of kprobes, see
- * Documentation/kprobes.txt
+ * Documentation/staging/kprobes.rst
  *
  * You will see the trace data in /var/log/messages and on the console
  * whenever _do_fork() is invoked to create a new process.

samples/kprobes/kretprobe_example.c

@@ -11,7 +11,7 @@
  * If no func_name is specified, _do_fork is instrumented
  *
  * For more information on theory of operation of kretprobes, see
- * Documentation/kprobes.txt
+ * Documentation/staging/kprobes.rst
  *
  * Build and insert the kernel module as done in the kprobe example.
  * You will see the trace data in /var/log/messages and on the console