Filling and indentation, please document

[Lennart Borgman]

There is a very nice feature of fill-paragraph that let you fill like this:

- This part will be filled
  and indented. Very nice.

But how to you do to make it fill this way? Is it documented? I used
to believe that putting point after "-" at "T" before calling
fill-paragraph made this happen, but that does not seem to be true now
(checked out 2009-12-04). Was I just dreaming that, or?