Non-nil if the property makes the text invisible.
POS-OR-PROP can be a marker or number, in which case it is taken to be
a position in the current buffer and the value of the `invisible' property
is checked; or it can be some other value, which is then presumed to be the
value of the `invisible' property of the text of interest.
The non-nil value returned can be t for truly invisible text or something
else if the text is replaced by an ellipsis. (fn POS-OR-PROP)