[cast] Fix safety comment bug for CastUnsized (#2908)

The decorated `unsafe impl` block is still sound, but the proof
previously proved too much (namely, that the cast preserves referent
size, when in fact it may shrink referent size).

gherrit-pr-id: Gba34c175d9c27344e336de4ed088eb0754d87f30

Author: joshlf

src/pointer/mod.rs

@@ -139,9 +139,10 @@ pub mod cast {
     // - Both sized and equal in size
     // - Both slice DSTs with the same trailing slice offset and element size
     //   and with align_of::<Src>() >= align_of::<Dst>(). These ensure that any
-    //   given pointer metadata encodes the same size for both `Src` and `Dst`
-    //   (note that the alignment is required as it affects the amount of
-    //   trailing padding).
+    //   given pointer metadata encodes the same size or more size for `Src`
+    //   than for `Dst` (note that the alignment is required as it affects the
+    //   amount of trailing padding). Thus, `project` preserves or shrinks the
+    //   set of referent bytes.
     unsafe impl<Src, Dst> Project<Src, Dst> for CastUnsized
     where
         Src: ?Sized + KnownLayout,