The function returns a TEXT with the full name of the user in argument user_name.
Argument user_name is a string with a user name, not to be confused with the user's full name.
userFullName(text user_name) #Output: Text
The function returns a TEXT LIST with the full names of the users in argument user_names.
Argument user_names is a string list with user names, not to be confused with the users' full names.
userFullName(text list user_names) #Output: Text list