explain place alignment

src/behavior-considered-undefined.md

@@ -23,7 +23,8 @@ 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 [based on
+  an unaligned pointer].
 * 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 +101,20 @@ 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`).
 
+### Places based on unaligned pointers
+[based on an unaligned pointer]: #places-based-on-unaligned-pointers
+
+A place is said to be "based on an unaligned pointer" if the last `*` projection
+during place computation was performed on a pointer that was not aligned for its
+type. (If there is no `*` projection in the place expression, then this is
+accessing the field of a local and rustc will guarantee proper alignment.)
+
+For instance, if `ptr` has struct type `S` with a field `f` of type `u8`, but
+the alignment of `S` is 8, then `ptr` must be 8-aligned or else `(*ptr).f` is
+"based on an unaligned pointer". In other words, the alignment requirement
+derives from the type of the pointer that was dereferenced, *not* the type of
+the field that is being accessed.
+
 ### Dangling pointers
 [dangling]: #dangling-pointers