Clarify behaviour of fields-to-delete

[bruno]

Some things that should be clarified on the Record javadoc (or the Repository.update javadoc):

• What happens upon Repository.update() if a field is both present in the Record object and in the list of fields to the delete?
• If the fields-to-delete takes precedence, will the repository then remove the field from the (returned) Record object?
• Is the list of fields-to-delete cleared after Repository.update() ?

[bruno]

Let's first look into #93, and then expand the docs.

[evert]

Added a clarification to the javadoc of Record#getFieldsToDelete that deleting a field will take precedence over updating a field. r4722

Moving this issue to 1.1 so it can be updated further wrt the fields that are put in the returned object once #93 has been resolved.