Break line at point and indent, continuing comment if within one.
This indents the body of the continued comment
under the previous comment line.
This command is intended for styles where you write a comment per line,
starting a new comment (and terminating it if necessary) on each line.
If you want to continue one comment across several lines, use C-j.
If a fill column is specified, it overrides the use of the comment column
or comment indentation.
The inserted newline is marked hard if variable `use-hard-newlines' is true,
unless optional argument SOFT is non-nil. (fn &optional SOFT)