The Coding standards say:
If the data type of a parameter or return value is not obvious or expected to be of a special class or interface, it is recommended to specify the data type in the @param or @return directive:
and
Primitive data types, such as int or string, do not need to be specified. It is recommended to specify classes and interfaces. If the parameter or return value is an array, or (anonymous/generic) object, you can specify the type if it would add clarity to the documentation. If for any reason, a primitive data type needs to be specified, use the lower-case data type name, e.g. "array". Also, make sure to use the most general class/interface possible. E.g., document the return value of db_select() to be a SelectQueryInterface, not a particular class that implements SelectQuery.
There is a proposal to simply require data type for all @param and @return lines for consistency.
If we set this precedent, the Coding Standards should be updated to reflect the policy change.
Please discuss at http://drupal.org/node/1290258