Add a deprecation notice to the checkNotNull function.
@@ -15,6 +15,11 @@ custom external functions, see
### `checkNotNull(value)` {#checkNotNull}
+WARNING: `checkNotNull` has been replaced with the
+[non-null assertion operator (`!`)](expressions.md#nonnull-assertion) and is
+deprecated. Please use `!` going forward as we are removing the `checkNotNull`
+function.
+
Throws a runtime exception if the given value is `null` and returns the value
otherwise.
