Currently, terms under other taxonomies can be partially handled correctly by doc_tags if use extends class of BP_Docs_Taxonomy. However, some details not playing out nicely--
docs-filter deosn't reflect the other taxonomy, it only repeat doc_tags's filter box.
The 2nd Tag Cloud can refelct the other taxonomy, if click a term, can get correct URL and redirect to correct tab, but can't find the doc.
The above 2 aspects is difficult to fix in extends class unless Doc provides an argument at places where it uses "$this->docs_tag_tax_name"