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

Continuation string to insert for multiline comments.
This string will be added at the beginning of each line except the very
first one when commenting a region with a commenting style that allows
comments to span several lines.
It should generally have the same length as `comment-start' in order to
preserve indentation.
If it is nil a value will be automatically derived from `comment-start'
by replacing its first character with a space.