spec: More precise wording in section on function calls.
A caller is not always in a function.
For example, a call can appear in top level declarations.
e.g. var x = f()
Change-Id: I29c4c3b7663249434fb2b8a6d0003267c77268cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/290849 Reviewed-by: Rob Pike <r@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Griesemer <gri@golang.org>