[DOCS] Synchronous logical replication?

devel version of the manual (2017-06-25 12:17:02) contains the following
paragraph documenting the CREATE SUBSCRIPTION command:

synchronous_commit (enum)

    ....

    A different setting might be appropriate when doing synchronous
    logical replication. The logical replication workers report the
    positions of writes and flushes to the publisher, and when using
    synchronous replication, the publisher will wait for the actual
    flush. This means that setting synchronous_commit for the subscriber
    to off when the subscription is used for synchronous replication
    might increase the latency for COMMIT on the publisher. In this
    scenario, it can be advantageous to set synchronous_commit to local
    or higher.

It seems to me quite confusing.
1) I failed to find in the docs whether logical replication supports
   synchronous mode at all and how it is configured.
2) Assuming it is supported, how setting synchronous_commit to off on
   replica (subscriber) can increase COMMIT latency on master?
   Obviously synchronous mode itself does increase it, but
     - In physical replication (docs for synchronous_commit in section
       19.5.1) only master's synchronous_commit value determines what
       it waits for (e.g. 'remote_write' means writing to replica's OS),
       and it seems natural to expect the same behaviour from logical
       replication
     - Even if replica's synchronous_commit somehow influences this,
       'off' value is the lowest and fastest, isn't it?


Arseny Sher
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company