bpf, doc: Do not link to docs.kernel.org for kselftest link

The question on how to run BPF selftests have a reference link to kernel
selftest documentation (Documentation/dev-tools/kselftest.rst). However,
it uses external link to the documentation at kernel.org/docs (aka
docs.kernel.org) instead, which requires Internet access.

Fix this and replace the link with internal linking, by using :doc: directive
while keeping the anchor text.

Fixes: b7a27c3 ("bpf, doc: howto use/run the BPF selftests")
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230228074523.11493-2-bagasdotme@gmail.com

Author: bagasme

Documentation/bpf/bpf_devel_QA.rst

@@ -461,8 +461,8 @@ needed::
 
   $ sudo make run_tests
 
-See the kernels selftest `Documentation/dev-tools/kselftest.rst`_
-document for further documentation.
+See :doc:`kernel selftest documentation </dev-tools/kselftest>`
+for details.
 
 To maximize the number of tests passing, the .config of the kernel
 under test should match the config file fragment in
@@ -688,7 +688,5 @@ when:
 .. _netdev-FAQ: Documentation/process/maintainer-netdev.rst
 .. _selftests:
    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/bpf/
-.. _Documentation/dev-tools/kselftest.rst:
-   https://www.kernel.org/doc/html/latest/dev-tools/kselftest.html
 
 Happy BPF hacking!