Update reserved keywords idl.adoc

Author: ShashankVM

arch/prose/idl.adoc

@@ -96,6 +96,7 @@ description   builtin
 body          for
 if            else
 enum          bitfield
+struct
 ----
 
 == Data Types
@@ -108,6 +109,7 @@ IDL has the following types:
  * Composite
  ** Enumerations
  ** Bitfields
+ ** Structs
  ** Arrays
  * Other
  ** Strings (with limited operators, mostly for configuration parameter checking)