Set WINDOW's redisplay end trigger value to VALUE.
WINDOW must be a live window and defaults to the selected one. VALUE
should be a buffer position (typically a marker) or nil. If it is a
buffer position, then if redisplay in WINDOW reaches a position beyond
VALUE, the functions in `redisplay-end-trigger-functions' are called
with two arguments: WINDOW, and the end trigger value. Afterwards the
end-trigger value is reset to nil. (fn WINDOW VALUE)