Inconsistencies in the *_users_to_group functions and description

Hi, 
This pull request corrects some inconsistencies detected in the affect_users_to_group function:
Substituted the invalid variable $cohort->id to $group->id in st->setCourse($group->id);
Removed invalid call to method error (from the class EnrolRecord) substituted by assigning the error string to $st-> error

Corrected description of the parameters in the functions * _users_to_group in wspp/clients/classes/MoodleWS.php