Update/create documentation for "priming the oplog" to include setting minValid

SERVER-7652 describes a change going into 2.2.3 in which initial sync criteria check minValid, not the oplog.

In cases in which we need to manually "prime the oplog" by inserting a document into a new oplog, e.g. when provisioning a secondary from a 'mongodump --oplog' and then 'mongorestore --oplogReplay' (SERVER-6080 gives a very brief overview), we now also need to set minValid. If minValid is not set, all data will be dropped and an initial sync done.

I cannot find anything on provisioning a secondary via mongodump/mongorestore in our official documentation. I am not sure if this is a deliberate choice or an omission. Given the impact of not setting minValid, we should consider documenting this procedure, including of course the need for the minValid.

By the way, manually setting of minValid is not needed when we change the size of the oplog, and as such that documentation does not need updating. (Tested on 2.3.2.)