bytes, strings: improve documentation for Count functions

Fixes #9659.

Change-Id: If364d5984a0c9a48858ae524b1560f633e621826
Reviewed-on: https://go-review.googlesource.com/3181
Reviewed-by: Rob Pike <r@golang.org>

src/bytes/bytes.go

@@ -23,7 +23,7 @@ func equalPortable(a, b []byte) bool {
 	return true
 }
 
-// explode splits s into a slice of UTF-8 sequences, one per Unicode character (still slices of bytes),
+// explode splits s into a slice of UTF-8 sequences, one per Unicode code point (still slices of bytes),
 // up to a maximum of n byte slices. Invalid UTF-8 sequences are chopped into individual bytes.
 func explode(s []byte, n int) [][]byte {
 	if n <= 0 {
@@ -47,6 +47,7 @@ func explode(s []byte, n int) [][]byte {
 }
 
 // Count counts the number of non-overlapping instances of sep in s.
+// If sep is an empty slice, Count returns 1 + the number of Unicode code points in s.
 func Count(s, sep []byte) int {
 	n := len(sep)
 	if n == 0 {

src/strings/strings.go

@@ -78,6 +78,7 @@ func hashStrRev(sep string) (uint32, uint32) {
 }
 
 // Count counts the number of non-overlapping instances of sep in s.
+// If sep is an empty string, Count returns 1 + the number of Unicode code points in s.
 func Count(s, sep string) int {
 	n := 0
 	// special cases