All documentation is included with the software and accessible from the Eclipse Help: choose Help > Help Contents from the menu and then select Functional Programming > Haskell from the tree at the left side. (Note for Linux users: if the Help is not displayed, you probably have to name a browser first in the preferences: go to Window > Preferences > Help and specify one.)
To get familiar with the main features of the environment, you can also try the cheat sheet: choose Help > Cheat Sheets from the menu and then select Functional Programming > Getting Started with Haskell in Eclipse.