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>