content: elaborate on function values

The description was a little bit sparse.

The next slide talks about closures, but since the idea of
function values wasn't clear to all readers, some people got
confused when the next slide defined a function that had
a function as a return value.

Fixes golang/go#4284

Change-Id: Id71f71cafb7c411dbad8306980f7eb1d75aa59e5
Reviewed-on: https://go-review.googlesource.com/15752
Reviewed-by: Andrew Gerrand <adg@golang.org>
2 files changed