Merge pull request #982 from mcclure/undefined-cast

Explicitly note any cast not defined in the documentation is illegal.

Author: Havvy

src/expressions/operator-expr.md

@@ -306,6 +306,7 @@ fn average(values: &[f64]) -> f64 {
 ```
 
 `as` can be used to explicitly perform [coercions](../type-coercions.md), as well as the following additional casts.
+Any cast that does not fit either a coercion rule or an entry in the table is a compiler error.
 Here `*T` means either `*const T` or `*mut T`.
 
 | Type of `e`           | `U`                   | Cast performed by `e as U`       |