Add a warning to my first blog post, letting the attentive reader know that
`emit_no_wait` doesn't exist any more, and linking them to what they should
be using if they want to do their own custom messages.
Makes sense to update all the docs to reflect the work done in #1738 but I
feel it doesn't quite make sense to retrofit this into an old blog post --
especially if the code it is referring to was like that at the time and
likely still will be for a wee while after this gets republished.
