content: clarify meaning of 'export'

The slide was already explaining what 'export' means.
Rather than restating it, this adds a bit more context about what
export means by contrasting it with what unexported means.

Fixes golang/go#12005

Change-Id: Id06daca3a519096e5c329f199c8db67f938cd463
Reviewed-on: https://go-review.googlesource.com/15556
Reviewed-by: Andrew Gerrand <adg@golang.org>
1 file changed