Documentation database_api.list_comments

• order: by_cashout_time show error when there is only one of two optional values
• order: by_root root_author and root_permlink are not required (but if one is blank, they must both be blank)
• order: by_root child_author and child_permlink are optional but if one is blank, they must both be blank
• order: by_parent parent_author parent_permlink are not required (but if one is blank, they must both be blank)
• order: by_parent child_author, child_permlink are optional but if one is blank, they must both be blank
• there are two more order by_update and by_author_last_update