make safety comments more explicit

Author: TyPR124

src/libstd/sys/windows/os_str.rs

@@ -78,15 +78,15 @@ impl Buf {
 
     pub fn as_slice(&self) -> &Slice {
         // Safety: Slice is just a wrapper for Wtf8,
-        // and as_slice returns &Wtf8. Therefore,
-        // transmuting &Wtf8 to &Slice is safe.
+        // and self.inner.as_slice() returns &Wtf8.
+        // Therefore, transmuting &Wtf8 to &Slice is safe.
         unsafe { mem::transmute(self.inner.as_slice()) }
     }
 
     pub fn as_mut_slice(&mut self) -> &mut Slice {
         // Safety: Slice is just a wrapper for Wtf8,
-        // and as_mut_slice returns &mut Wtf8. Therefore,
-        // transmuting &mut Wtf8 to &mut Slice is safe.
+        // and self.inner.as_mut_slice() returns &mut Wtf8.
+        // Therefore, transmuting &mut Wtf8 to &mut Slice is safe.
         unsafe { mem::transmute(self.inner.as_mut_slice()) }
     }