If t incremental search can match hidden text.
A nil value means don't match invisible text.
When the value is `open', if the text matched is made invisible by
an overlay having an `invisible' property and that overlay has a property
`isearch-open-invisible', then incremental search will show the contents.
(This applies when using `outline.el' and `hideshow.el'.)
See also `reveal-mode' if you want overlays to automatically be opened
whenever point is in one of them.