Documentation for the route builder

[xabbuh]

see symfony/symfony#15778

[weaverryan]

This is partially included in the micro kernel docs: #5899. I'm not really sure what else we need to show, most of it is kind of php-doc level stuff (what methods can I call, what arguments are there).

[javiereguiluz]

I agree with Ryan. The useful part is documented and the rest is probably too internal. Let's close.