Slight rephrasing

Author: mcol

src/reference-manual/blocks.Rmd

@@ -565,7 +565,8 @@ After the generated quantities statements are executed, the constraints
 on the declared generated quantity variables are validated.
 
 All variables declared as generated quantities are printed as part of
-the output, unless they are declared in a nested block. For example:
+the output.  Variables declared in nested blocks are local variables,
+not generated quantities, and thus won't be printed.  For example:
 ```stan
 generated quantities {
   int a; // added to the output