Update function.dd

Added explanation of less well-known abbreviation IDL.

function.dd

@@ -789,7 +789,7 @@ int foo(in int x, out int y, ref int z, int q);
         $(UL
         $(LI The function declaration makes it clear what the inputs and
         outputs to the function are.)
-        $(LI It eliminates the need for IDL as a separate language.)
+        $(LI It eliminates the need for IDL (interface description language) as a separate language.)
         $(LI It provides more information to the compiler, enabling more
         error checking and
         possibly better code generation.)