slices: clarify MinFunc/MaxFunc result for equal elements
They should return the first of equal elements. No such clarification
is required for Min/Max as for them equal elements are indistinguishable.
For #60091
Change-Id: Iad58115d482add852c811e993131702b5b3bec5e
Reviewed-on: https://go-review.googlesource.com/c/go/+/505796
Run-TryBot: Ian Lance Taylor <iant@golang.org> Reviewed-by: Eli Bendersky <eliben@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>