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.
I am open to better wording. They are callbacks provided for a continuation, and the begin method is set up to synchronize the calls before calling the next step or finishing, this is what I am trying to describe here.

I am also open to removing this section as well, but to me it seems like a good place to breadcrumb someone who is trying to figure out how Mojo::IOLoop's delays work and how to use them. In a lot of cases the delay helper may be what they should be using instead, I think "delayed rendering" is a good phrase to refer to that use case.