slices: document that BinarySearch[Func] return earliest position

Fixes #65446

Change-Id: I08dc512fb1f0101eb8aac8767cdf582360699559
Reviewed-on: https://go-review.googlesource.com/c/go/+/562345
Reviewed-by: Eli Bendersky <eliben@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/slices/sort.go

@@ -117,10 +117,10 @@ func MaxFunc[S ~[]E, E any](x S, cmp func(a, b E) int) E {
 	return m
 }
 
-// BinarySearch searches for target in a sorted slice and returns the position
-// where target is found, or the position where target would appear in the
-// sort order; it also returns a bool saying whether the target is really found
-// in the slice. The slice must be sorted in increasing order.
+// BinarySearch searches for target in a sorted slice and returns the earliest
+// position where target is found, or the position where target would appear
+// in the sort order; it also returns a bool saying whether the target is
+// really found in the slice. The slice must be sorted in increasing order.
 func BinarySearch[S ~[]E, E cmp.Ordered](x S, target E) (int, bool) {
 	// Inlining is faster than calling BinarySearchFunc with a lambda.
 	n := len(x)