Applied the suggestion in PR #198.

CppCoreGuidelines.md

@@ -2536,7 +2536,7 @@ A `not_null<T*>` is assumed not to be the `nullptr`; a `T*` may be the `nullptr`
 
 ##### Reason
 
-Clarity. Making it clear that a test for null isn't needed.
+Clarity. A function with a `not_null<T>` parameter makes it clear that the caller of the function is responsible for any `nullptr` checks that may be necessary. Similarly, a function with a return value of `not_null<T>` makes it clear that the caller of the function does not need to check for `nullptr`.
 
 ##### Example