Display MESSAGE in the echo area if possible, otherwise in a pop-up buffer.
MESSAGE may be either a string or a buffer.
A buffer is displayed using `display-buffer' if MESSAGE is too long for
the maximum height of the echo area, as defined by `max-mini-window-height'
if `resize-mini-windows' is non-nil.
Returns either the string shown in the echo area, or when a pop-up
buffer is used, the window used to display it.
If MESSAGE is a string, then the optional argument BUFFER-NAME is the
name of the buffer used to display it in the case where a pop-up buffer
is used, defaulting to `*Message*'. In the case where MESSAGE is a
string and it is displayed in the echo area, it is not specified whether
the contents are inserted into the buffer anyway.
Optional arguments NOT-THIS-WINDOW and FRAME are as for `display-buffer', and only used if a buffer is displayed.