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

vlib/encoding/csv/writer.v

@@ -13,6 +13,7 @@ pub mut:
 	delimiter u8
 }
 
+// new_writer returns a reference to a Writer
 pub fn new_writer() &Writer {
 	return &Writer{
 		delimiter: `,`
@@ -75,6 +76,7 @@ fn (w &Writer) field_needs_quotes(field string) bool {
 	return false
 }
 
+// str returns the writer contents
 pub fn (mut w Writer) str() string {
 	return w.sb.str()
 }