Make the current definition and/or comment visible.
Further invocations move it to the top of the window or toggle the
visibility of comments that precede it.
Point is left unchanged unless prefix ARG is supplied.
If the definition is fully onscreen, it is moved to the top of the
window. If it is partly offscreen, the window is scrolled to get the
definition (or as much as will fit) onscreen, unless point is in a comment
which is also partly offscreen, in which case the scrolling attempts to get
as much of the comment onscreen as possible.
Initially `reposition-window' attempts to make both the definition and
preceding comments visible. Further invocations toggle the visibility of
the comment lines.
If ARG is non-nil, point may move in order to make the whole defun
visible (if only part could otherwise be made so), to make the defun line
visible (if point is in code and it could not be made so, or if only
comments, including the first comment line, are visible), or to make the
first comment line visible (if point is in a comment). (fn &optional ARG)