New replication behavior for capped collections needs to be documented.

The replication behavior of capped collections has changed. By default, capped collections do not have an _id index. Prior to 2.2, this caused replication of capped collections to be very slow, until the user created the _id index themselves. In 2.2, the _id index is still not created by default on the primary, but it is created on the secondaries after some number of insert or updates. This prevents replication from falling behind. We still recommend that users create the _id index manually on the primary, so that if the primary node fails over and becomes a secondary, it will already have an _id index.