Update the upstream documentation landing page.

This page previously was pointing to the old kselftest/test branch,
which (a) is no longer used, and (b) isn't even where we're getting the
mirrored documentation.

Replaced it with a link to the torvalds master branch, which is where
the documentation mirror is copied from.

I've also linked to the kselftest/kunit branch, which is where most
development happens, and to the kunit documentation 'latest' version on
kernel.org as a mirror.

Change-Id: I8d25487e049c5e0867498ace32f90ce880b3395f

third_party/kernel/index.rst

diff --git a/third_party/kernel/index.rst b/third_party/kernel/index.rst
index 05d1bdb..a735f69 100644
--- a/third_party/kernel/index.rst
+++ b/third_party/kernel/index.rst
@@ -9,9 +9,15 @@
 Where to get the code
 ---------------------
 
-https://git.kernel.org/pub/scm/linux/kernel/git/shuah/linux-kselftest.git/log/?h=test
+This documentation is taken from torvalds' master branch:
+https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/
+
+Most KUnit upstream development is done against the 'kunit' branch of the linux kselftest repository:
+https://git.kernel.org/pub/scm/linux/kernel/git/shuah/linux-kselftest.git/log/?h=kunit
 
 Getting Started Guide
 ---------------------
 
 `docs/ <docs/>`_
+
+This documentation is also `mirrored on kernel.org <https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html>`_.