WRITING-931: rephrases production cluster arch toc description

Signed-off-by: Allison Moore <allison.moore@10gen.com>
WRITING-921: adds mention of TTL to indexes intro

Signed-off-by: Allison Moore <allison.moore@10gen.com>
WRITING-928: fixes example to actually return results

Signed-off-by: Allison Moore <allison.moore@10gen.com>
WRITING-924: adds link to geo tutorial in seealso

Signed-off-by: Allison Moore <allison.moore@10gen.com>
WRITING-897: clarify what sparse does in unique index tutorial

Signed-off-by: Allison Moore <allison.moore@10gen.com>
edits with Michael's feedback

Signed-off-by: Allison Moore <allison.moore@10gen.com>