doc: clarify non-nil zero length slice to array pointer conversion
doc: clarify non-nil zero length slice to array pointer conversion

There is an example for nil slice already, so adding example for non-nil
zero length slice, too, clarifying to the reader that the result is also
non-nil and different from nil slice case.

Updates #395

Change-Id: I019db1b1a1c0c621161ecaaacab5a4d888764b1a
Reviewed-on: https://go-review.googlesource.com/c/go/+/336890
Trust: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
Trust: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Run-TryBot: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

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