Document Pu128.

And move the `repr` line after the `derive` line, where it's harder to
overlook. (I overlooked it initially, and didn't understand how this
type worked.)

compiler/rustc_data_structures/src/packed.rs

@@ -3,8 +3,10 @@
 use std::cmp::Ordering;
 use std::fmt;
 
-#[repr(packed(8))]
+/// A packed 128-bit integer. Useful for reducing the size of structures in
+/// some cases.
 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq, PartialOrd, Ord)]
+#[repr(packed(8))]
 pub struct Pu128(pub u128);
 
 impl Pu128 {