How does the Laravel 4.1 documentation work?

I've gotten a start with how Laravel works from its docs as well as from tutorials and SO. But I keep running into the same problem when I try to use their API/class reference.

For example, I've been able to use the URL class like this:

URL::to('string')

Which I learnt about from a tutorial. But if I look at the documentation for Illuminate\Support\Facades\URL it doesn't list a to() method.

Conversely, if I look at the documentation for Illuminate\Filesystem\Filesystem and I try to call the get() method like this:

$file = new Filesystem;
$file->get('lorem.txt'); 

I get the following error

Class 'Filesystem' not found

My questions:

• How do I know which Classes I can use and which Methods I can call?
• Where do I find this information?
• Or am I just missing something about how Laravel works?