add subsection aboyt empty arrays

Author: avehtari

src/reference-manual/expressions.qmd

@@ -334,8 +334,22 @@ array[2, 3] int b = { { 1, 2, 3 },
                 { 4, 5, 6 } };
 ```
 
+#### Empty arrays {-}
+
 The empty array expression (`{ }`) is not allowed. See more about restrictions on array expressions in subsection [Restrictions on values](#array-expressions).
 
+If needed, it is possible to create an empty array with
+```stan
+rep_array(e, 0)
+```
+where the first expression `e` determines the type of the array. For
+example, `rep_array(0.0, 0)` returns an empty real array of type
+`real[]`, whereas `rep_array({123}, 0)` returns an empty two
+dimensional integer array of type `int[ , ]`. Only the type of the
+first argument is used, so the integer arrays `{123}` and `{0}`
+produce equivalent values.
+
+
 ### Array expression types {-}
 
 Any type of expression may be used within braces to form an array
@@ -418,13 +432,6 @@ 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