DOCS-871 multithreaded replication
minor copy edits to tag aware sharding docs
minor: style changes
merge: DOCS-871
DOCS-871: edits
DOCS-387 FAQ entry for shard balancer prioritization

DOCS-387 incorporate Jeremy's comments

DOCS-387 balancer and shard drain
merge: DOCS-387
minor: punctuation
DOCS-871: edits
DOCS-699-RELATED update link
merge: DOCS-699
mixed-cluster uppgrade warning
DOCS-1146: typo fix
remove errant comma
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