docs: move the kref doc into the core-api book

This document covers core kernel objects. So, add it into the core-api
book.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/f385af13b4a6d3ff8c89beedd4506900e79ca72e.1588345503.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/core-api/index.rst

@@ -31,6 +31,7 @@ Library functionality that is used throughout the kernel.
    :maxdepth: 1
 
    kobject
+   kref
    assoc_array
    xarray
    idr

Documentation/core-api/kobject.rst

@@ -210,7 +210,7 @@ statically and will warn the developer of this improper usage.
 If all that you want to use a kobject for is to provide a reference counter
 for your structure, please use the struct kref instead; a kobject would be
 overkill.  For more information on how to use struct kref, please see the
-file Documentation/kref.txt in the Linux kernel source tree.
+file Documentation/core-api/kref.rst in the Linux kernel source tree.
 
 
 Creating "simple" kobjects