Scroll text of selected window down ARG lines.
If ARG is omitted or nil, scroll down by a near full screen.
A near full screen is `next-screen-context-lines' less than a full screen.
Negative ARG means scroll upward.
If ARG is the atom `-', scroll upward by nearly full screen.
When calling from a program, supply as argument a number, nil, or `-'.
(fn &optional ARG)