minor 2.4 patch edits

Signed-off-by: Sam Kleinman <samk@10gen.com>
Update replica-sets.txt

chainingAllowed backported to 2.2.2
Update control-access-to-mongodb-with-authentication.txt

Typos and grammar.
Update control-access-to-mongodb-with-authentication.txt

typographical changes
DOCS-1254 powerOf2 textindex and DOCS-1247 geoWithin in agg