https://docs.python.org/3/tutorial/controlflow.html#function-annotations
is the direct link.

As Mark said, 'ham' is a required positional-or-keyword argument.
'eggs' is an optional 'positional-or-keyword.  The sentence as is is wrong, even if Robert garbled the reason.  With the signature as it, the sentence should be "The following example has a required argument, an optional argument, and the return value annotated."

To make the existing sentence true, the signature could be changed to
  def f(ham: str, /, *, eggs: str) -> str:
But I think this would be wrong.  Annotation does not depend on how an argument is passed, but whether it has a default (making it optional).  In particular, it shows that when optional, the annotation goes *before* '= default', not after.  (It could have gone after: eggs = 'eggs': str.)