DOC: Add docstrings for two methods in primary.py
DOC: Add docstrings for two methods in primary.py

refresh_toplevel_metadata_from_repositories
and
get_target_list_from_director

Also include a missed error in an <Exceptions> summary

uptane/uptane,awwad/uptane,awwad/uptane,shikhar394/uptane,shikhar394/uptane,uptane/uptane