#37 Improve ContainerInterface::has() specification

Author: mnapoli

docs/ContainerInterface.md

@@ -34,6 +34,8 @@ Users of dependency injections containers (DIC) are referred to as `user`.
 
 - `has` takes one unique parameter: an entry identifier. It MUST return `true`
   if an entry identifier is known to the container and `false` if it is not.
+  `has($id)` returning true does not mean that `get($id)` will not throw an exception.
+  It does however mean that `get($id)` will not throw a `NotFoundException`.
 
 ### 1.2 Exceptions