Attachments were added in WP 2, whereas conditional tags as a whole have been around since version 1.5. So is_attachment() was apparently overlooked in the documentation.
is_paged() can test any posts query for whether it is paginated (typically through use of the posts_nav_link() template tag). It works on multiple posts queries such as you get in a category, dated archive, or the home page. The use of <!--nextpage-->
to create pagination on a single post or Page does not work with it. No real contradiction, as WordPress looks at the two types of pagination differently.