bug#29222: 26.0.90; doc string of `search-invisible'

[Eli Zaretskii]

> Date: Wed, 8 Nov 2017 21:00:29 -0800 (PST)
> From: Drew Adams <drew.adams@oracle.com>
> 
> This part of the doc string:
> 
>  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.
> 
> would be clearer if it said "an overlay having a non-nil 'invisible'
> property" and "...has a non-nil property `isearch-open-invisible'.

Thanks, done on the release branch.