Add @method PHPDocs for the message wrapped functions handled in __call

[caricare2]

While cleaning up our codebase, we noticed a lot of warnings from PHPStan such as this:

Call to an undefined method Maknz\Slack\Message::send().                         
         method.notFound        

This appears to be a side effect of the magic wrapping of various methods through __call to getOrCreateMessage and then return $this. For example code like this causes warnings because it doesn't recognize the methods that are chained:

        $this->client->to($slackChannel)->attach([
            'title' => $slackTitle,
            'text'       => $slackMessage,
            'color'      => 'danger'
        ])->send($slackTitle);

Adding PHPDocs to the Client class for the more common methods to hint that these methods exist would help, such as this:

/**
 * @method self to(string $channel)
 * @method self attach(mixed $attachment)
 */