note on not-too-large slices

Author: RalfJung

src/what-unsafe-does.md

@@ -50,8 +50,10 @@ 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. As a consequence, if the span is
 empty, "dangling" is the same as "non-null". Note that slices point to their
-entire range, so it's very important that the length metadata is never too
-large. If for some reason this is too cumbersome, consider using raw pointers.
+entire range, so it's very important that the length metadata is never too large
+(in particular, allocations and therefore slices cannot be bigger than
+`isize::MAX` bytes). If for some reason this is too cumbersome, consider using
+raw pointers.
 
 That's it. That's all the causes of Undefined Behavior baked into Rust. Of
 course, unsafe functions and traits are free to declare arbitrary other