Clarify docs on what traversal category allowed

[jesseli2002]

Thanks to @sebrockm for clarifying for me that the traversal category can inherit from more than one tag. In case anyone has the same question as me in the future (#301), I've updated the docs to make this explicit.

Also, as noted in #301 I couldn't find any reference to vertex_and_edge_list_tag, so I've removed it from the documentation.

[jeremy-murphy]

Thanks! It's little improvements like this that make the library easier for everyone to use.