Doc - additional 3.4 related info (#7645)

Documentation/Books/Manual/ReleaseNotes/UpgradingChanges34.md

@@ -611,6 +611,11 @@ For arangod, the following startup options have changed:
 
   The default value for this option is *false*.
 
+  As mentioned above, ArangoDB 3.4 changes the default value of the configuration option 
+  `--rocksdb.total-write-buffer-size` to about 40% of available physical RAM, and 512MiB
+  for setups with less than 4GiB of RAM. In ArangoDB 3.3 this option had a default value
+  of `0`, which meant that the memory usage for write buffers was not limited.
+
 
 Permissions
 -----------
@@ -650,6 +655,25 @@ should change the protocol from SSLv2 to TLSv12 if possible, by adjusting
 the value of the `--ssl.protocol` startup option.
 
 
+Replication
+-----------
+
+By default, database-specific and global replication appliers use a slightly
+different configuration in 3.4 than in 3.3. In 3.4 the default value for the
+configuration option `requireFromPresent` is now `true`, meaning the follower
+will abort the replication when it detects gaps in the leader's stream of 
+events. Such gaps can happen if the leader has pruned WAL log files with 
+events that have not been fetched by a follower yet, which may happen for 
+example if the network connectivity between follower and leader is bad.
+
+Previous versions of ArangoDB 3.3 used a default value of `false` for 
+`requireFromPresent`, meaning that any such gaps in the replication data 
+exchange will not cause the replication to stop. 3.4 now stops replication by
+default and writes according errors to the log. Replication can automatically
+be restarted in this case by setting the `autoResync` replication configuration
+option to `true`.
+
+
 Mixed-engine clusters
 ---------------------
 
@@ -714,6 +738,8 @@ This change will be visible only on systems which allow assigning names to
 threads.
 
 
+
+
 Deprecated features
 ===================