Improve documentation on api.bulkDocs

The current documentation for [api.bulkDocs](https://github.com/daleharvey/pouchdb/blob/master/docs/api.md#create-a-batch-of-documents) makes no mention of a 'new_edits' option, which is pretty important for replication: [http://wiki.apache.org/couchdb/HTTP_Bulk_Document_API#Posting_Existing_Revisions](http://wiki.apache.org/couchdb/HTTP_Bulk_Document_API#Posting_Existing_Revisions)

I'll grab this in a bit.