See https://jira.mongodb.org/browse/DOCS-2908. In mmapv1 changes are visible before the journal is forced to disk even when j:true is used on the write because this is how mmapv1 behaves. But I assume that MongoDB+WiredTiger does not release row/doc locks until the redo log is written, and if j:true is also set then changes are not visible until they are durable.

If true, then the docs should be updated and see https://jira.mongodb.org/browse/DOCS-2908 for pointers to the docs for mmapv1.