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* Also trim the IMAGE_CLASSES entry which was quite outdated.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>

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