Clarify the required semantics for broadcasting with __setitem__ (#429)

Closes gh-424

Author: asmeurer

spec/API_specification/broadcasting.rst

@@ -112,4 +112,17 @@ The following examples demonstrate array shapes which do **not** broadcast.
 In-place Semantics
 ------------------
 
-As implied by the broadcasting algorithm, in-place element-wise operations must not change the shape of the in-place array as a result of broadcasting.
+As implied by the broadcasting algorithm, in-place element-wise operations (including ``__setitem__``) must not change the shape of the in-place array as a result of broadcasting. Such operations should only be supported in the case where the right-hand operand can broadcast to the shape of the left-hand operand, after any indexing operations are performed.
+
+For example:
+
+::
+
+   x = empty((2, 3, 4))
+   a = empty((1, 3, 4))
+
+   # This is OK. The shape of a, (1, 3, 4), can broadcast to the shape of x[...], (2, 3, 4)
+   x[...] = a
+
+   # This is not allowed. The shape of a, (1, 3, 4), can NOT broadcast to the shape of x[1, ...], (3, 4)
+   x[1, ...] = a