The Moodle documentation won't be terribly helpful, since this is more of a server issue than a Moodle issue. Also, setting up these options varies wildly depending on whether you're using Windows or Linux, and even which flavour of Linux you're using.
This really does depend on your host. If it's shared hosting, you're probably screwed. Changes like this affect an entire server, so it's very unlikely your host will change the server configuration just for you. I would bet large sums of money against it.
If it's a virtual server or a dedicated server and you have command line access to the server, it can be done quite easily. If it's Linux, just use your distribution's package manager to install those PHP extensions.
In Ubuntu, you'd want
sudo apt-get install php5-xmlrpc
sudo apt-get install php5-intl
PHP won't notice you've installed those extensions until you restart Apache.
sudo service apache2 restart