minor: reword "stable_kernel/index.rst" to not call alpha/master "stable"

We arguably go and want to rename all these files, but that's a more
involved change.

Change-Id: I7b6878413a0d5bbac778ab119d484a1f9a033bc9
Signed-off-by: Daniel Latypov <dlatypov@google.com>
diff --git a/third_party/stable_kernel/index.rst b/third_party/stable_kernel/index.rst
index fab76f0..003faa9 100644
--- a/third_party/stable_kernel/index.rst
+++ b/third_party/stable_kernel/index.rst
@@ -1,10 +1,13 @@
 =================================
-Stable Version (aka Not Upstream)
+Deprecated Version ("kunit/alpha/master")
 =================================
 
-Here we have a copy of the documentation taken from the stable branch of KUnit.
+Here we have a copy of the documentation taken from the old "alpha" version
+KUnit.
 
-For convenience:
+You should really prefer using the upstream version if possible, but the alpha
+version has some features that still haven't made their way upstream (class
+mocking, feature mocking, death tests, etc.)
 
 Where to get the code
 ---------------------