expand server push

[bagder]

When I do HTTP/2 presentations I get a lot of questions about server push so I think it is worthwhile to expand that section and describe:

1 - the client can reject them at once if it wants to (and it has a small initial window)
2 - the contents will not end up in the browser cache immediately unless the resource is otherwise used
3 - no, a server cannot DOS a client with this in anyway more than it already could using other means