Added tests and documentation for soa<> rate qualifier.

tests_errors/soa-9.ispc

@@ -0,0 +1,9 @@
+// Can't convert from pointer to SOA type "soa<8> struct A * uniform" to pointer to non-SOA type "void * varying" 
+
+struct A { float a, b; };
+
+soa<8> A as[100];
+
+void foo() {
+    void *ptr = &as[0];
+}