Fix comment about alignments

Author: oli-obk

src/librustc/mir/interpret/value.rs

@@ -46,8 +46,8 @@ pub enum ConstValue<'tcx> {
     /// An allocation together with a pointer into the allocation.
     /// Invariant: the pointer's `AllocId` resolves to the allocation.
     /// The alignment exists to allow `const_field` to have `ByRef` access to nonprimitive fields
-    /// of packed structs. The alignment may be lower than the alignment of the `Allocation` and
-    /// allow reads with lower alignment than what the allocation would normally permit.
+    /// of packed structs. The alignment may be lower than the type of this constant.
+    /// This permits reads with lower alignment than what the type would normally require.
     /// FIXME(RalfJ,oli-obk): The alignment checks are part of miri, but const eval doesn't really
     /// need them. Disabling them may be too hard though.
     ByRef(Pointer, Align, &'tcx Allocation),