docs: update description of updateLog / autoCommit in solrconfig

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1433817 13f79535-47bb-0310-9956-ffa450edef68

solr/example/solr/collection1/conf/solrconfig.xml

@@ -304,6 +304,16 @@
   <!-- The default high-performance update handler -->
   <updateHandler class="solr.DirectUpdateHandler2">
 
+    <!-- Enables a transaction log, used for real-time get, durability, and
+         and solr cloud replica recovery.  The log can grow as big as
+         uncommitted changes to the index, so use of a hard autoCommit
+         is recommended (see below).
+         "dir" - the target directory for transaction logs, defaults to the
+                solr data directory.  --> 
+    <updateLog>
+      <str name="dir">${solr.ulog.dir:}</str>
+    </updateLog>
+ 
     <!-- AutoCommit
 
          Perform a hard commit automatically under certain conditions.
@@ -319,8 +329,11 @@
                    since a document was added before automaticly
                    triggering a new commit. 
          openSearcher - if false, the commit causes recent index changes
-         to be flushed to stable storage, but does not cause a new
-         searcher to be opened to make those changes visible.
+           to be flushed to stable storage, but does not cause a new
+           searcher to be opened to make those changes visible.
+
+         If the updateLog is enabled, then it's highly recommended to
+         have some sort of hard autoCommit to limit the log size.
       -->
      <autoCommit> 
        <maxTime>15000</maxTime> 
@@ -370,14 +383,6 @@
        </listener>
       -->
 
-    <!-- Enables a transaction log, currently used for real-time get.
-         "dir" - the target directory for transaction logs, defaults to the
-            solr data directory.  --> 
-    <updateLog>
-      <str name="dir">${solr.ulog.dir:}</str>
-    </updateLog>
-   
-
   </updateHandler>
   
   <!-- IndexReaderFactory