strings: revise ToUpperSpecial and ToLowerSpecial wording

Fixes #26654

Change-Id: I4832c45cad40607b83e1a8a9b562fa12e639b7d9
GitHub-Last-Rev: c9ceedb7d4b4c01f91ea4fe3dc3496e73eed9120
GitHub-Pull-Request: golang/go#26781
Reviewed-on: https://go-review.googlesource.com/127716Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/strings/strings.go

@@ -616,13 +616,13 @@ func ToLower(s string) string {
 func ToTitle(s string) string { return Map(unicode.ToTitle, s) }
 
 // ToUpperSpecial returns a copy of the string s with all Unicode letters mapped to their
-// upper case, giving priority to the special casing rules.
+// upper case using the case mapping specified by c.
 func ToUpperSpecial(c unicode.SpecialCase, s string) string {
 	return Map(func(r rune) rune { return c.ToUpper(r) }, s)
 }
 
 // ToLowerSpecial returns a copy of the string s with all Unicode letters mapped to their
-// lower case, giving priority to the special casing rules.
+// lower case using the case mapping specified by c.
 func ToLowerSpecial(c unicode.SpecialCase, s string) string {
 	return Map(func(r rune) rune { return c.ToLower(r) }, s)
 }