tweak: clarify the available is the default read concern for secondary reads if not causal consistent sessions

Author: kay-kim

source/administration/production-checklist-development.txt

@@ -98,27 +98,30 @@ Sharding
    - Use :ref:`targeted operations <sharding-mongos-targeted>`
      for workloads that need to scale with the number of shards.
 
-   - For MongoDB 3.4 and earlier, read from the primary nodes for
+   - **For MongoDB 3.4 and earlier**, read from the primary nodes for
      :ref:`non-targeted or broadcast <sharding-mongos-broadcast>`
      queries as these queries may be sensitive to `stale or orphaned
      data
      <http://blog.mongodb.org/post/74730554385/background-indexing-on-se condaries-and-orphaned>`_.
 
-   - For MongoDB 3.6 and later, secondaries no longer return orphaned
-     data unless using read concern :readconcern:`"available"`. All
-     members of the shard replica set maintain chunk metadata, allowing
-     them to filter out orphans when not using
-     :readconcern:`"available"`. As such, :ref:`non-targeted or
-     broadcast <sharding-mongos-broadcast>` queries that are not using
-     :readconcern:`"available"` can be safely run on any member and
-     will not return orphaned data.
-
-     The :readconcern:`"available"` read concern can return
-     :term:`orphaned documents <orphaned document>` from secondary
-     members since it does not check for updated chunk metadata.
-     However, if the return of orphaned documents is immaterial to an
-     application, the :readconcern:`"available"` read concern provides
-     the lowest latency reads possible among the various read concerns.
+   - | **For MongoDB 3.6 and later**, secondaries no longer return orphaned
+       data unless using read concern :readconcern:`"available"` (which
+       is the default read concern for reads against secondaries when not
+       associated with :ref:`causally consistent sessions <sessions>`).
+
+     | Starting in MongoDB 3.6, all members of the shard replica set
+       maintain chunk metadata, allowing them to filter out orphans
+       when not using :readconcern:`"available"`. As such,
+       :ref:`non-targeted or broadcast <sharding-mongos-broadcast>`
+       queries that are not using :readconcern:`"available"` can be
+       safely run on any member and will not return orphaned data.
+
+     | The :readconcern:`"available"` read concern can return
+       :term:`orphaned documents <orphaned document>` from secondary
+       members since it does not check for updated chunk metadata.
+       However, if the return of orphaned documents is immaterial to an
+       application, the :readconcern:`"available"` read concern provides
+       the lowest latency reads possible among the various read concerns.
 
    - :doc:`Pre-split and manually balance chunks
      </tutorial/create-chunks-in-sharded-cluster>` when inserting large