Improve the explanation of how to find the span.

Author: Lokathor

reference/src/glossary.md

@@ -27,13 +27,19 @@ In this case, both `r` and `s` alias each other, since they both point to all of
 the bytes of `u`.
 
 However, `head` and `tail` do not alias each other: `head` points to the first
-byte of `u` and `tail` points to the other seven bytes of `u` after it. Also,
-both `head` and `tail` alias `s`.
-
-* The span length of `&T`, `&mut T`, `*const T`, or `*mut T` when `T` is
-  [`Sized`](https://doc.rust-lang.org/core/marker/trait.Sized.html) is
-  `size_of<T>()`.
-* When `T` is not `Sized` the span length is `size_of_val(t)`.
+byte of `u` and `tail` points to the other seven bytes of `u` after it. Both `head`
+and `tail` alias `s`, any overlap is sufficient to count as an alias.
+
+* The span of a pointer or reference is the size of the value being pointed to or referenced.
+* For some type `T` that is [`Sized`](https://doc.rust-lang.org/core/marker/trait.Sized.html)
+  The span length of a pointer or reference to `T` is found with `size_of::<T>()`.
+* When `T` is not `Sized` the story is a little tricker:
+  * If you have a reference `r` you can use `size_of_val(r)` to determine the
+    span of the reference.
+  * If you have a pointer `p` you must unsafely convert that to a reference before
+    you can use `size_of_val`. There is not currently a safe way to determine the
+    span of a pointer to an unsized type.
+* The Data Layout chapter also has more information on the sizes of different types.
 
 One interesting side effect of these rules is that references and pointers to
 Zero Sized Types _never_ alias each other, because their span length is always 0
@@ -123,4 +129,4 @@ niche.
 * *tag*
 * *rvalue*
 * *lvalue*
-* *representation*
+* *representation*