Improve docs about return only types

Void and never are types that can only be used for function return
types. Officially php doesn't have a category of types that are only
allowed for return, but I think it makes sense to name them like this.

Author: jaapio

src/Types/Never_.php

@@ -16,7 +16,7 @@
 use phpDocumentor\Reflection\Type;
 
 /**
- * Value Object representing the pseudo-type 'never'.
+ * Value Object representing the return-type 'never'.
  *
  * Never is generally only used when working with return types as it signifies that the method that only
  * ever throw or exit.

src/Types/Void_.php

@@ -16,7 +16,7 @@
 use phpDocumentor\Reflection\Type;
 
 /**
- * Value Object representing the pseudo-type 'void'.
+ * Value Object representing the return-type 'void'.
  *
  * Void is generally only used when working with return types as it signifies that the method intentionally does not
  * return any value.