Clarify what gets printed in the generated quantities block

Author: mcol

src/reference-manual/blocks.Rmd

@@ -565,4 +565,13 @@ 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.
+the output, unless they are declared in a nested block. For example:
+```stan
+generated quantities {
+  int a; // added to the output
+
+  {
+    int b; // not added to the output
+  }
+}
+```