Suggested changes for FindAndModify

• We should document how findAndModify is different from update and show why this operator different
• Find and modify has a significant performance impact, we should mention this. Ideally with benchmarks
• We mention that this is an "Atomic" operator. This can lead to users incorrectly believing that this is the only atomic update operator. We should clarify that both are atomic as part of explaining what is different about findAndModify vs update. We should likely reference http://docs.mongodb.org/manual/tutorial/isolate-sequence-of-operations/ as well.