Update documentation for classes split

* Make reference to meta/classes* instead of just meta/classes
* Update each reference to the path where the class has moved
* Add a brief explanation to the classes reference explaining the split
* Add a note to INHERIT, INHERIT_DISTRO, IMAGE_CLASSES and USER_CLASSES
  variable glossary entries mentioning where the specified classes must
  be located. I contemplated adding the same note to PACKAGE_CLASSES
  but decided against it, as it has a very specific usage and such a
  note might distract from the narrative of that entry.
* Also trim the IMAGE_CLASSES entry which was quite outdated.
* INHERIT += no longer works with testimage so drop the note about that

(From yocto-docs rev: e76bed00b452d3049f6c22afbfb980b557a141bd)

Signed-off-by: Paul Eggleton <paul.eggleton@microsoft.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/dev-manual/common-tasks.rst

@@ -973,7 +973,7 @@ a recipe and using :term:`EXTRA_IMAGE_FEATURES` from within your
 :term:`Build Directory`.
 
 To understand how these features work, the best reference is
-:ref:`meta/classes/image.bbclass <ref-classes-image>`.
+:ref:`meta/classes-recipe/image.bbclass <ref-classes-image>`.
 This class lists out the available
 :term:`IMAGE_FEATURES` of which most map to package groups while some, such
 as ``debug-tweaks`` and ``read-only-rootfs``, resolve as general
@@ -6889,7 +6889,7 @@ multiple times if you have more than one set of modules to package.
 For more examples that show how to use ``do_split_packages``, see the
 ``connman.inc`` file in the ``meta/recipes-connectivity/connman/``
 directory of the ``poky`` :ref:`source repository <overview-manual/development-environment:yocto project source repositories>`. You can
-also find examples in ``meta/classes/kernel.bbclass``.
+also find examples in ``meta/classes-recipe/kernel.bbclass``.
 
 Following is a reference that shows ``do_split_packages`` mandatory and
 optional arguments::