Require Definitions for New SGR-2 Stats

Per: https://issues.couchbase.com/browse/CBG-910

Statistics definitions were recently updated (see: https://issues.couchbase.com/browse/DOC-6500).

So, I have text for those and some of the SGR statistics items too. But, notwithstanding their apparently obvious meaning, I don't have definitions for the following 'new' SGR-2 statistics. 

• sgr_num_attachments_pushed
• sgr_num_attachment_bytes_pushed
• sgr_num_attachments_pulled
• sgr_num_attachment_bytes_pulled
• sgr_num_docs_pulled
• sgr_num_docs_purged
• sgr_num_docs_failed_to_pull
• sgr_push_conflict_count
• sgr_push_rejected_count
• sgr_docs_checked_recv
• sgr_deltas_recv
• sgr_conflict_resolved_local_count
• sgr_conflict_resolved_remote_count
• sgr_conflict_resolved_merge_count

Definitions are therefore required for these items and for any others introduced and-or relocated in 2.8.