DOCS-1742: amount of space used by compaction
findAndModify edits
copy edit findAndModify subheading
DOCS-1743: correcting sources of confusion for mongodump/mongorestore with replica set members
Update expire-data.txt

Clarifying how Mongo handles documents under a TTL index, that omit the indexed column.

Signed-off-by: Sam Kleinman <samk@10gen.com>
reorg: aggregation framework