serialization: Remove "changeset" from terminology
serialization: Remove "changeset" from terminology

On Wed, Aug 31, 2016 at 06:53:45PM -0700, Brandon Philips wrote [1]:
> @wking I think changeset is the logical concept, layer is the
> serialization of that changeset into a tar. I really don't have a
> strong opnion about having different nouns for these things but
> @stevvooe seems to.

On Thu, Sep 01, 2016 at 10:29:16AM -0700, Stephen Day wrote [2]:
> A "changeset" is just an english word for a set of changes. We
> represent a set of changes with a layer.

That is generic enough that we don't need to formally define
"changeset", so I've removed the term entry.

Also update the layer entry to:

* Link to the new layer.md.
* Be more specific about what a set of layers is.  It's a filesystem,
  not the whole image (which also contains configuration, etc.).
* Mention that a layer is a specific tar-based serialization of the
  abstract "set of filesystem changes" idea.

[1]: https://github.com/opencontainers/image-spec/pull/231#issuecomment-243954395
[2]: https://github.com/opencontainers/image-spec/pull/231#issuecomment-244151555

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

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