Document size_of for 128-bit integers

We might want to consider separately documenting the alignment of
primitives, rather than just their size, since 128-bit integers, unlike
all other primitives, have an alignment that is not identical to their
size (size_of is 16, align_of is 8)

src/libcore/mem.rs

@@ -194,10 +194,12 @@ pub fn forget<T>(t: T) {
 /// u16 | 2
 /// u32 | 4
 /// u64 | 8
+/// u128 | 16
 /// i8 | 1
 /// i16 | 2
 /// i32 | 4
 /// i64 | 8
+/// i128 | 16
 /// f32 | 4
 /// f64 | 8
 /// char | 4