clarify that references size_of_val can never exceed isize::MAX

Author: RalfJung

src/behavior-considered-undefined.md

@@ -78,11 +78,12 @@ reading uninitialized memory is permitted are inside `union`s and in "padding"
 A reference/pointer is "dangling" if it is null or not all of the bytes it
 points to are part of the same allocation (so in particular they all have to be
 part of *some* allocation). The span of bytes it points to is determined by the
-pointer value and the size of the pointee type (using `size_of_val`). As a
-consequence, if the span is empty, "dangling" is the same as "non-null". Note
-that slices and strings point to their entire range, so it is important that the length
-metadata is never too large. In particular, allocations and therefore slices and strings
-cannot be bigger than `isize::MAX` bytes.
+pointer value and the size of the pointee type (using `size_of_val`).
+
+Note that dynamically sized types (such as slices and strings) point to their
+entire range, so it is important that the length metadata is never too large. In
+particular, the dynamic size of a Rust value (as determined by `size_of_val`)
+must never exceed `isize::MAX`.
 
 [`bool`]: types/boolean.md
 [`const`]: items/constant-items.md