The Guide for the Force.com Migration Tool is prob. the least well maintained documentation for the platform. I've never seen so many inaccurate items in any other documents (search "Migration" on my blog for more). Here's one I kept forgetting to write about: the WebLink entity, which is meant for the custom links used on page layouts. Note the capital L in the name, which in the Guide is lower case ("Weblink"). Using lowercase will give you "Entity type: 'Weblink' is unknown", as the manifest file is very much properly XML, i.e., case sensitive.