DOCS-871 multithreaded replication
merge: DOCS-871
DOCS-871: edits
merge: DOCS-387
minor: punctuation
DOCS-871: edits
DOCS-699-RELATED update link
merge: DOCS-699
mixed-cluster uppgrade warning
DOCS-1146: typo fix
merge: minor edit
DOCS-1042 and DOCS-935 backup system clarifications and improvements
DOCS-788: adding example of a DBRef
DOCS-967: adding clarification about uptime in config database
DOCS-550 cross linking to noscripting option
DOCS-896: note about local.slaves
DOCS-1076: note regarding repair and atomicity
add note about not mixing forms

please cleanup style/formatting
DOCS-1150 editing note about $query operator
minor: whitespace and paragraph changes