Update required for links to documentation in help and API messages

To improve SEO, the MongoDB documentation URL has recently changed. 

Old URL: www.docs.mongodb.com

New URL: www.mongodb.com/docs 

There are at least a few instances where the Mongo Shell help and API messages provide a link directly to additional information in the docs. While any links with the old URL structure will be redirected indefinitely, it would be good to update the links for consistency.