bytes, strings: document that FieldsFunc f must not be stateful

Fixes #8738.

LGTM=adg
R=golang-codereviews, adg
CC=golang-codereviews
https://golang.org/cl/143260045

src/bytes/bytes.go

@@ -267,6 +267,8 @@ func Fields(s []byte) [][]byte {
 // It splits the slice s at each run of code points c satisfying f(c) and
 // returns a slice of subslices of s.  If all code points in s satisfy f(c), or
 // len(s) == 0, an empty slice is returned.
+// FieldsFunc makes no guarantees about the order in which it calls f(c).
+// If f does not return consistent results for a given c, FieldsFunc may crash.
 func FieldsFunc(s []byte, f func(rune) bool) [][]byte {
 	n := 0
 	inField := false

src/strings/strings.go

@@ -347,6 +347,8 @@ func Fields(s string) []string {
 // FieldsFunc splits the string s at each run of Unicode code points c satisfying f(c)
 // and returns an array of slices of s. If all code points in s satisfy f(c) or the
 // string is empty, an empty slice is returned.
+// FieldsFunc makes no guarantees about the order in which it calls f(c).
+// If f does not return consistent results for a given c, FieldsFunc may crash.
 func FieldsFunc(s string, f func(rune) bool) []string {
 	// First count the fields.
 	n := 0