docs: document more builtin functions/methods (#14229)

vlib/encoding/utf8/utf8.v

@@ -7,10 +7,12 @@ mut:
 	failed   bool
 }
 
+// validate_str reports if str consists of valid UTF-8 runes
 pub fn validate_str(str string) bool {
 	return validate(str.str, str.len)
 }
 
+// validate reports if data consists of valid UTF-8 runes
 pub fn validate(data &u8, len int) bool {
 	mut state := Utf8State{}
 	for i := 0; i < len; i++ {