doc: improve the interface example in Effective Go

The String method is n-squared and overwrites its receiver.
Fix both issues, with only a slight loss of clarity.

Fixes #28773

Change-Id: I588f69d4cbd72931b28b984671512834473bd466
Reviewed-on: https://go-review.googlesource.com/c/151217Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/effective_go.html

@@ -2106,12 +2106,14 @@ In this contrived example <code>Sequence</code> satisfies both.
 
 <p>
 The <code>String</code> method of <code>Sequence</code> is recreating the
-work that <code>Sprint</code> already does for slices.  We can share the
-effort if we convert the <code>Sequence</code> to a plain
+work that <code>Sprint</code> already does for slices.
+(It also has complexity O(N²), which is poor.) We can share the
+effort (and also speed it up) if we convert the <code>Sequence</code> to a plain
 <code>[]int</code> before calling <code>Sprint</code>.
 </p>
 <pre>
 func (s Sequence) String() string {
+    s = s.Copy()
     sort.Sort(s)
     return fmt.Sprint([]int(s))
 }
@@ -2138,6 +2140,7 @@ type Sequence []int
 
 // Method for printing - sorts the elements before printing
 func (s Sequence) String() string {
+    s = s.Copy()
     sort.IntSlice(s).Sort()
     return fmt.Sprint([]int(s))
 }

doc/progs/eff_sequence.go

@@ -28,11 +28,18 @@ func (s Sequence) Swap(i, j int) {
 	s[i], s[j] = s[j], s[i]
 }
 
+// Copy returns a copy of the Sequence.
+func (s Sequence) Copy() Sequence {
+	copy := make(Sequence, 0, len(s))
+	return append(copy, s...)
+}
+
 // Method for printing - sorts the elements before printing.
 func (s Sequence) String() string {
+	s = s.Copy() // Make a copy; don't overwrite argument.
 	sort.Sort(s)
 	str := "["
-	for i, elem := range s {
+	for i, elem := range s { // Loop is O(N²); will fix that in next example.
 		if i > 0 {
 			str += " "
 		}