[10.x] Allow Expression at more places in Query Builder

[pascalbaljet]

The code below causes PHPStan errors because the second argument is an Expression instance. The instance will eventually be transformed in Grammer::whereColumn(), so this should be allowed.

DB::table('users', 't1')->leftJoin('users AS t2', DB::raw('t1.uid + 1'), '=', 't2.uid');

I added the Expression class to more docblocks to solve this. This relates to #46160