[12.x] Specify that the query builder returns instances of stdClass

[axlon]

The query builder returns query results as instances of stdClass, however currently it is documented as if it can return any object. This PR narrows the type to more accurately reflect what is being returned. This doesn't affect calling code, editors, etc. but it does change something for PHPStan.

PHPStan has a special list of classes on which it assumes any property can exist, it calls this universal object crates, stdClass is one of these classes, this means that:

Before:

$result = $query->find($id); // <- object|null

if (is_null($result)) {
    return;
}

echo "Hello, $result->first_name"; // <- PHPStan error, first_name might not exist on 'object'

After:

$result = $query->find($id); // <- stdClass|null

if (is_null($result)) {
    return;
}

echo "Hello, $result->first_name"; // <- OK

Relevant PHPStan docs: https://phpstan.org/config-reference#universal-object-crates

Note: Some query builder methods (get, cursor) already use stdClass

[axlon]

Failing test seems unrelated (this PR only changes docs)