The  parameter is an alias for  - I tend to prefer the specific to the general, so for newspapers, I normally use . Now, regarding the documentation that you linked, it says
 publisher: Name of publisher; may be wikilinked if relevant. The publisher is the company that publishes the work being cited. Do not use the publisher parameter for the name of a work (e.g., a book, encyclopedia, newspaper, magazine, journal, website). Not normally used for periodicals. Corporate designations such as "Ltd", "Inc" or "GmbH" are not usually included. Omit where the publisher's name is substantially the same as the name of the work (for example, The New York Times Co. publishes The New York Times newspaper, so there is no reason to name the publisher). 
In the case of The Observer, the company that publishes the work being cited is Guardian News and Media Limited - not The Guardian. This publication could be cited as  . However, for The Guardian - where the publisher's name is substantially the same as the name of the work - we would use  alone.