slices: explicitly document nil and empty slice equality

Fixes golang#68472

Change-Id: I21282621d679d9e2b032256b338c0b5ff107c8f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/599816
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

src/slices/slices.go

@@ -15,6 +15,7 @@ import (
 // elements equal. If the lengths are different, Equal returns false.
 // Otherwise, the elements are compared in increasing index order, and the
 // comparison stops at the first unequal pair.
+// Empty and nil slices are considered equal.
 // Floating point NaNs are not considered equal.
 func Equal[S ~[]E, E comparable](s1, s2 S) bool {
 	if len(s1) != len(s2) {