slices: document that Clone can return extra capacity

Change-Id: I8b9aa1f36ce554d8855ee70e15f0dd0a8cf1097a
Reviewed-on: https://go-review.googlesource.com/c/go/+/568342
Reviewed-by: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>

Author: ianlancetaylor

src/slices/slices.go

@@ -342,6 +342,7 @@ func Replace[S ~[]E, E any](s S, i, j int, v ...E) S {
 
 // Clone returns a copy of the slice.
 // The elements are copied using assignment, so this is a shallow clone.
+// The result may have additional unused capacity.
 func Clone[S ~[]E, E any](s S) S {
 	// The s[:0:0] preserves nil in case it matters.
 	return append(s[:0:0], s...)