[#2750] Rewrite tag vocabularies docs

Change the tag vocabularies docs to refer to the working example in
ckanext/example_idatasetform instead of providing their own detailed
instructions.

Also add some more general explanation at the start about what tag
vocabularies are for/why you might want to use them/what you can do with
them, it was all a bit mysterious.

Remove some false claims from the Properties section, I don't think that
the restricted/free vocabulary types are implemented (they are either
always restricted or always free, can't remember), vocabularies do not
have descriptions.
[#2750] example_idatasetform use toolkit not logic

Use ckan.plugins.toolkit instead of ckan.logic in ckanext/example_idatasetform