math: document that Min/Max differ from min/max

math: document that Min/Max differ from min/max

For #59488
Fixes #60616

Change-Id: Idf9f42d7d868999664652dd7b478684a474f1d96
Reviewed-on: https://go-review.googlesource.com/c/go/+/501355
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>