Update promotion rules

src/reference-manual/expressions.Rmd

@@ -613,10 +613,10 @@ a ? b : c
 ```
 
 is an expression of the promoted type of `b` and `c`.  The
-only promotion allowed in Stan is from integer to real; if one
+only promotion allowed in Stan is integer -> real -> complex; e.g. if one
 argument is of type `int` and the other of type `real`, the
-conditional expression as a whole is of type `real`.  In all
-other cases, the arguments have to be of the same underlying Stan type
+conditional expression as a whole is of type `real`.
+In other cases, the arguments have to be of the same underlying Stan type
 (i.e., constraints don't count, only the shape) and the conditional
 expression is of that type.
 
@@ -1070,7 +1070,7 @@ to complex expressions.
 
 ### Promotion
 
-There are two promotion rules.  First, integer expressions of type
+There are three promotion rules.  First, integer expressions of type
 `int` may be used anywhere an expression of type `real` is used.  An
 integer is promoted to real by casting it in the underlying C++.
 
@@ -1083,7 +1083,9 @@ Promotion is transitive, so that integers may be promoted to complex
 numbers in two stages, first converting the integer to real, then
 converting the real value to a complex type.
 
-
+The third rule is (starting with Stan 2.29), array types are promoted
+according to their contained type. For example, `array[,] int` may be
+used where `array [,] real` or `array [,] complex` is required.
 
 #### Literals {-}
 

src/reference-manual/syntax.Rmd

@@ -375,8 +375,9 @@ quantities blocks.
 
 The types of expressions used for elements in array expressions
 (`'{' expressions '}'`) must all be of the same type or a mixture
-of `int` and `real` types (in which case the result is
-promoted to be of type `real`).
+of scalar (`int`, `real` and `complex`) types (in which case the result
+is promoted to be of the highest type on the  `int -> real -> complex`
+hierarchy).
 
 
 ### Forms of numbers {-}