WIP: Expand docs on unstructuring

[td-anne]

I ran into some confusing situations trying to unstructure some objects I had defined (#513 ) and it was suggested I create a PR clarifying the docs. This PR attempts to do this.

I should say, my understanding of how cattrs works, and of how cattrs is meant to be used, is a little shaky, so feedback on this documentation is highly welcome.

I am also totally new to cattrs development, so I'm not sure where anything goes. In particular, is there a place for demo code that will be executed? Should I add tests to verify that the behaviour I am describing in the docs is really what happens? I'm not sure how to run the doctests, particularly in the docs/ directory.