reflect: document that StructOf panics on unexported fields

Fixes #25401

Change-Id: I0b61ecfcee43ebfe0a84b5c1e28a3817f96b94ed
Reviewed-on: https://go-review.googlesource.com/115015Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/reflect/type.go

@@ -2345,8 +2345,9 @@ func isValidFieldName(fieldName string) bool {
 // The Offset and Index fields are ignored and computed as they would be
 // by the compiler.
 //
-// StructOf currently does not generate wrapper methods for embedded fields.
-// This limitation may be lifted in a future version.
+// StructOf currently does not generate wrapper methods for embedded
+// fields and panics if passed unexported StructFields.
+// These limitations may be lifted in a future version.
 func StructOf(fields []StructField) Type {
 	var (
 		hash       = fnv1(0, []byte("struct {")...)