Clarify usage of $delay->begin and provide links between delay docs

The documentation of Mojo::IOLoop::Delay->begin can use some clarification to better explain its usage and effects. Also, Mojo::IOLoop->delay can indicate how it should be used by referring to begin and the delay helper.
We don't use the phrase "synchronized continuation callbacks" anywhere else in the documentation, and it confuses me a little. Can you elaborate on where exactly you got it from and why it would be appropriate here?