explain place alignment

Author: RalfJung

src/behavior-considered-undefined.md

@@ -23,7 +23,7 @@ code.
 </div>
 
 * Data races.
-* Accessing (loading from or storing to) a place that is [dangling] or unaligned.
+* Accessing (loading from or storing to) a place that is [dangling] or [unaligned].
 * Performing a place projection that violates the requirements of [in-bounds
   pointer arithmetic][offset]. A place projection is a [field
   expression][project-field], a [tuple index expression][project-tuple], or an
@@ -100,6 +100,13 @@ reading uninitialized memory is permitted are inside `union`s and in "padding"
 
 The span of bytes a pointer or reference "points to" is determined by the pointer value and the size of the pointee type (using `size_of_val`).
 
+### Unaligned places
+[unaligned]: #unaligned-places
+
+The alignment requirement of a place is computed based on the entire place expression since the last `*` (if any).
+If the place was computed via `(*ptr).field.field`, then the final place is considered aligned only if `ptr` is aligned *according to its type*.
+For instance, if a field of type `u8` sits at offset 4 in an 8-aligned struct, then loading from `(*ptr).field` requires alignment 4.
+
 ### Dangling pointers
 [dangling]: #dangling-pointers