comment-multi-line is a variable defined in `newcomment.el'.
Its value is

Non-nil means `comment-indent-new-line' continues comments.
That is, it inserts no new terminator or starter.
This affects `auto-fill-mode', which is the main reason to
customize this variable.

It also affects C-M-j. However, if you want this
behavior for explicit filling, you might as well use M-x newline-and-indent.

You can customize this variable.