Complete output for get_taxonomy in the documentation

[XavierBeguin]

The current documentation on the tera function get_taxonomy does not mention either lang or permalink fields, that are nonetheless returned by the function as the following template extract shows:

{%- set taxonomy_config = get_taxonomy(kind="tags", lang="en") %}
<ul>
    <li>taxonomy_config.lang = {{ taxonomy_config.lang }}</li>
    <li>taxonomy_config.kind = {{ taxonomy_config.kind }}</li>
    <li>taxonomy_config.permalink = {{ taxonomy_config.permalink }}</li>
    <li>taxonomy_config.items = {{ taxonomy_config.items }}</li>
</ul>

This PR simply adds the two fields to the type of output for get_taxonomy in the documentation.

[Keats]

Thanks!