spec: corrections to various sections

- fix definition of "specific types" and add more examples
- state that a parameterized function must be instantiated
  when used as a function value
- remove duplicate word ("can can" -> "can")

Thanks to @danscales for finding these.

Change-Id: Ideb41efc35a3e67694d3bc97e462454feae37c44
Reviewed-on: https://go-review.googlesource.com/c/go/+/367394


Trust: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>