DS-1951  Global Setting to stop Explicit Authentication Trigger for GET-DSObject requests

added comment in dspace/config/modules/authentication.cfg
to explain what this does 

```
# set the auto.explicit = false  
# IF using implicit authentication mechanism, like IP Authentication,  as well 
# as explicit authentication, eg PasswordAuthentication
# AND 
# IF you want DSPACE to return a 500 code instead of prompting users to supply credentials 
# to explicit authentication methods,  when user issues request to a community, collection,
# item, # bitstreams URL that is implicitly protected and for which the request fails to 
# pass the implicit authorization test 
#
# default is true - DSPACE prompts for credentials 
auto.explicit = true
```