Scroll next window upward ARG lines; or near full screen if no ARG.
A near full screen is `next-screen-context-lines' less than a full screen.
The next window is the one below the current one; or the one at the top
if the current one is at the bottom. Negative ARG means scroll downward.
If ARG is the atom `-', scroll downward by nearly full screen.
When calling from a program, supply as argument a number, nil, or `-'.
If `other-window-scroll-buffer' is non-nil, scroll the window
showing that buffer, popping the buffer up if necessary.
If in the minibuffer, `minibuffer-scroll-window' if non-nil
specifies the window to scroll. This takes precedence over
`other-window-scroll-buffer'. (fn &optional ARG)