Add upgrade notes for 3.x to 3.4.x

The default node name in configure.ps1 changed in #4678, from couchdb@localhost to couchdb@127.0.0.1

The same change in default node name (and resulting behaviour) also occurred in "When upgrading from 2.x to 2.1.1", so the upgrade notes for 3.4.x have been updated with the same text.

https://docs.couchdb.org/en/stable/whatsnew/2.1.html#upgrade-notes

Author: doctor-claw

src/docs/src/whatsnew/3.4.rst

@@ -20,6 +20,24 @@
     :depth: 1
     :local:
 
+.. _release/3.4.x/upgrade:
+
+Upgrade Notes
+=============
+
+* When upgrading from 3.x to 3.4.x, if you have not customized your
+  node name in ``vm.args``, be sure to retain your original ``vm.args``
+  file. The default node name has changed from ``couchdb@localhost`` to
+  ``couchdb@127.0.0.1``, which can prevent CouchDB from accessing existing
+  databases on the system. You may also change the name option back to the
+  old value by setting ``-name couchdb@localhost`` in ``etc/vm.args`` by
+  hand. The default has changed to meet new guidelines and to provide
+  additional functionality in the future.
+
+  If you receive errors in the logfile, such as
+  ``internal_server_error : No DB shards could be opened.`` or in Fauxton,
+  such as ``This database failed to load.`` you need to make this change.
+
 .. _release/3.4.2:
 
 Version 3.4.2