Fix WhereClause::whereExists() and WhereClause::whereNotExists() PHPDoc

[nikolaystrikhar]

PHPStan throws Call to an undefined method ::limit() when you do something like this. I checked and all builder methods there return $this, so I just did the same for these 2 methods.

DB::table( 'posts' )
    ->select( 'ID' )
    ->whereNotExists(
        function ( QueryBuilder $builder ) {
            // whatever.
        }
    )
    ->limit( 1 );

[nikolaystrikhar]

@estevao90 pinged me about it, so adding him to a conversation just in case :-)

[borkweb]

Released as 1.0.5!