ptr::RawPtr, spell out units used for the offset argument.

spell out units used for the `offset` argument, so that callers do not
try to scale to byte units themselves.

src/libstd/ptr.rs

@@ -213,7 +213,8 @@ pub trait RawPtr<T> {
     /// be pointing to invalid memory.
     unsafe fn to_option(&self) -> Option<&T>;
     /// Calculates the offset from a pointer. The offset *must* be in-bounds of
-    /// the object, or one-byte-past-the-end.
+    /// the object, or one-byte-past-the-end.  `count` is in units of T; e.g. a
+    /// `count` of 3 represents a pointer offset of `3 * sizeof::<T>()` bytes.
     unsafe fn offset(self, count: int) -> Self;
 }