That depends on plugins used. Current Eclipse version (Helios) indeed doesn't support this out the box (update: this is definitely supported since Kepler which is the first Eclipse version with builtin JSF 2.0 support).
When you're using for example Glassfish 3 and use the Glassfish server plugin and the project has Glassfish set as target runtime, then you'll see autocompletion and documentation for all tags and attributes in the Facelet XHTML page. You will even get an extra rightclick menu option on the project New > XHTML file.
Here are some screens of a Eclipse+Glassfish project:
Tag tooltip:
Tag autocompletion:
Attribute autocompletion:
When using for example Tomcat as target runtime, this feature is not available. Even more, syntax highlighting doesn't work at all. You would have to manually add *.xhtml extension to JSP editor settings to get the highlighting to work by Window > Preferences > General > Content Types > Text > JSP. But that's it.
For JBoss AS 6 you need the JBoss Tools plugin which can be installed as described here. This plugin by the way unfortunately doesn't work out for Tomcat.
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