[10.x] Fix Http client pool return type

[miguilimzero]

The current docblock code presumes that any method by PendingRequest return itself, however, this is not the case, even for simple scenarios:

Http::pool(fn (Pool $pool) => [
    $pool->get('http://localhost/first'), // Returns Promise and not PendingRequest
    $pool->get('http://localhost/second'), // Returns Promise and not PendingRequest
    $pool->get('http://localhost/third'), // Returns Promise and not PendingRequest
]);

Any method that represents creating an async request will return a Promise class: get, head, post, patch, put, and delete. Only methods to configure the request will return the class itself, ex: withOptions, retry, timeout.

When using a static analysis tool and creating a function to mock a pool request is where the problem comes. Here is an example to demonstrate it:

public function prepareMyPoolRequest(Pool $pool, string $endpoint): Promise // This is the correct return type, however when running Psalm, it will throw an error saying that the ->get() method returns a PendingRequest.
{
     $baseUrl = '...';
     
     return $pool->get($baseUrl.$endpoint);
}

This pull request adds \GuzzleHttp\Promise\Promise to the __call magic method from Pool class for async requests. This will create a union, and any method called (according to the new docblock) may return any of the two classes. This may generate some uncertainty in some static analysis cases.

A better way to patch this, in the static analysis view, would be creating the methods get, head, post, patch, put, and delete on the Pool class, enforcing that they will return a Promise instance and all other methods will fallback to the __call magic method that says it returns a PendingRequest instance, which is now correct. But this will lead to some new lines of code instead of just adjusting a docblock value.