Revert "DOCS-14180 replica sets should use setDefaultRWConcern to modify default write concern"

This reverts commit 7564c3b.

Author: andf-mongodb

source/core/replica-set-write-concern.txt

@@ -110,8 +110,21 @@ documentation on write concern, see :ref:`write-concern`.
 Modify Default Write Concern
 ----------------------------
 
-You can modify the default write concern for a replica set by issuing 
-the :dbcommand:`setDefaultRWConcern` command.
+.. TODO need to remove most of this section and rewrite what's left.
+   Post 2.6 though.
+
+You can modify the default write concern for a replica set by setting
+the :rsconf:`settings.getLastErrorDefaults` setting
+in the :doc:`replica set configuration
+</reference/replica-configuration>`. The following sequence of commands
+creates a configuration that waits for the write operation to complete
+on a majority of the voting members before returning:
+
+.. code-block:: javascript
+
+   cfg = rs.conf()
+   cfg.settings.getLastErrorDefaults = { w: "majority", wtimeout: 5000 }
+   rs.reconfig(cfg)
 
 If you issue a write operation with a specific write concern, the write
 operation uses its own write concern instead of the default.