Because the names are similar, it's easy to confuse document workspaces and document libraries, but they are very different.

A document workspace is a special type of subsite that contains works-in-progress.  Workspaces often contain only one document (perhaps a large report or spreadsheet) that a team is working on.

A document library is used to store multiple documents within a site.  Completed documents should be stored in document libraries that are part of the team site.  Document libraries are a special type of list (not a type of site).  Libraries collect related documents and share them with all members of the site.  Documents can be organized into folders within the library, allowing you to categorize them.

Every document workspace site actually includes a document library for the work-in-progress, but because document workspaces are subsites of the team site, not everyone has access to those files.  Storing completed documents in a library at the team site makes those documents more widely available.

You can't create document libraries from the Shared Workspace task pane--you do it by viewing the SharePoint site in a browser and choosing Create -> Document Library and completing the New Document Library form.  Of course, there's always an exception: you can create picture libraries using the task pane in Office Picture Manager.