Fill paragraph at or after point.
If JUSTIFY is non-nil (interactively, with prefix argument), justify as well.
If `sentence-end-double-space' is non-nil, then period followed by one
space does not end a sentence, so don't break a line there.
The variable `fill-column' controls the width for filling.
If `fill-paragraph-function' is non-nil, we call it (passing our
argument to it), and if it returns non-nil, we simply return its value.
If `fill-paragraph-function' is nil, return the `fill-prefix' used for filling.
The REGION argument is non-nil if called interactively; in that
case, if Transient Mark mode is enabled and the mark is active,
call `fill-region' to fill each of the paragraphs in the active region, instead of just filling the current paragraph.