Momentarily display STRING in the buffer at POS.
Display remains until next event is input.
If POS is a marker, only its position is used; its buffer is ignored.
Optional third arg EXIT-CHAR can be a character, event or event
description list. EXIT-CHAR defaults to SPC. If the input is
EXIT-CHAR it is swallowed; otherwise it is then available as
input (as a command if nothing else).
Display MESSAGE (optional fourth arg) in the echo area.
If MESSAGE is nil, instructions to type EXIT-CHAR are displayed there.