Return a window currently displaying BUFFER-OR-NAME, or nil if none.
BUFFER-OR-NAME may be a buffer or a buffer name and defaults to
the current buffer.
The optional argument ALL-FRAMES specifies the frames to consider:
- t means consider all windows on all existing frames.
- `visible' means consider all windows on all visible frames.
- 0 (the number zero) means consider all windows on all visible
and iconified frames.
- A frame means consider all windows on that frame only.
Any other value of ALL-FRAMES means consider all windows on the
selected frame and no others.
(fn &optional BUFFER-OR-NAME ALL-FRAMES)