Re-factor bp_docs_tab() wrap orphaned bp_docs_create_button() in li tags

Wraps the create  new anchor in list tags otherwise anchor is child of ul. Add function arg to allow template file to pass a show button 'false' condition to remove button from doc_tabs, defaults to true = 'show', wraps li/button in checks for 'user_can'. arg filter mainly a convenience for developers who may need control & want to echo the button elsewhere - changes don't take into account other screens or circumstances.