unicode/utf8: document the handling of runes out of range in EncodeRune

Document the way EncodeRune currently handles runes which are
out of range.  Also add an example showing that behaviour.

Change-Id: I0f8e7645ae053474ec319085a2bb6d7f73bc137c
Reviewed-on: https://go-review.googlesource.com/c/go/+/255998
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Giovanni Bajo <rasky@develer.com>
Trust: Giovanni Bajo <rasky@develer.com>
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/unicode/utf8/example_test.go

@@ -107,6 +107,26 @@ func ExampleEncodeRune() {
 	// 3
 }
 
+func ExampleEncodeRune_outOfRange() {
+	runes := []rune{
+		// Less than 0, out of range.
+		-1,
+		// Greater than 0x10FFFF, out of range.
+		0x110000,
+		// The Unicode replacement character.
+		utf8.RuneError,
+	}
+	for i, c := range runes {
+		buf := make([]byte, 3)
+		size := utf8.EncodeRune(buf, c)
+		fmt.Printf("%d: %d %[2]s %d\n", i, buf, size)
+	}
+	// Output:
+	// 0: [239 191 189] <20> 3
+	// 1: [239 191 189] <20> 3
+	// 2: [239 191 189] <20> 3
+}
+
 func ExampleFullRune() {
 	buf := []byte{228, 184, 150} // 世
 	fmt.Println(utf8.FullRune(buf))

src/unicode/utf8/utf8.go

@@ -337,6 +337,7 @@ func RuneLen(r rune) int {
 }
 
 // EncodeRune writes into p (which must be large enough) the UTF-8 encoding of the rune.
+// If the rune is out of range, it writes the encoding of RuneError.
 // It returns the number of bytes written.
 func EncodeRune(p []byte, r rune) int {
 	// Negative values are erroneous. Making it unsigned addresses the problem.