clarify empty array expression

Author: avehtari

src/reference-manual/expressions.qmd

@@ -334,6 +334,7 @@ array[2, 3] int b = { { 1, 2, 3 },
                 { 4, 5, 6 } };
 ```
 
+The empty array expression (`{ }`) is not allowed. See more about restrictions on array expressions in subsection [Restrictions on values](#array-expressions).
 
 ### Array expression types {-}
 
@@ -417,6 +418,13 @@ zero-element array.
 array[0] int a;   // a is fully defined as zero element array
 ```
 
+If needed, it is also possible to create an empty array with
+```stan
+rep_array(0, 0)
+```
+where the type of the first argument determines the type of the array
+(e.g., real or int).
+
 #### No zero-tuples or one-tuples {-}
 
 There is no way to declare or construct a zero-tuple or one-tuple in