Alt-Shift-J, IRC, or from the context menu. It won't add Javadocs automagically if you type in a function. If it's created from a template, like when overriding etc., you can define the template for the Javadoc as well.
I believe I used to have a tool that would do that, but I don't remember what it was. Jalopy might do it, and it's available as an Eclipse plugin.
You don't need to do a control-space to make autodoc comments work. Just type "/**" above the method/class definition and press ENTER. Eclipse will automatically close the comment and put in as much of the doc body as it can deduce. No control key press required.
An IDE is no substitute for an Intelligent Developer.