strings, bytes: clarify usage of EqualFolds

This clarifies meaning of "case folding" Unicode equality with more familiar "case insensitive" wording.
For case folding properties see ftp://ftp.unicode.org/Public/UNIDATA/CaseFolding.txt.

Fixes #33447

Change-Id: I6ee85ab398679bf2a0b7d18693985ff0979d6c5a
GitHub-Last-Rev: accc9159330c61e046d77f77beac62b38bf72c19
GitHub-Pull-Request: golang/go#34434
Reviewed-on: https://go-review.googlesource.com/c/go/+/196717Reviewed-by: Rob Pike <r@golang.org>

src/bytes/bytes.go

@@ -935,7 +935,8 @@ func ReplaceAll(s, old, new []byte) []byte {
 }
 
 // EqualFold reports whether s and t, interpreted as UTF-8 strings,
-// are equal under Unicode case-folding.
+// are equal under Unicode case-folding, which is a more general
+// form of case-insensitivity.
 func EqualFold(s, t []byte) bool {
 	for len(s) != 0 && len(t) != 0 {
 		// Extract first rune from each.

src/strings/strings.go

@@ -969,7 +969,8 @@ func ReplaceAll(s, old, new string) string {
 }
 
 // EqualFold reports whether s and t, interpreted as UTF-8 strings,
-// are equal under Unicode case-folding.
+// are equal under Unicode case-folding, which is a more general
+// form of case-insensitivity.
 func EqualFold(s, t string) bool {
 	for s != "" && t != "" {
 		// Extract first rune from each string.