docs(engine): add property isolation level property and update documentation

Related to camunda/camunda-bpm-platform#4516

Author: joaquinfelici

content/reference/deployment-descriptors/tags/process-engine.md

@@ -506,7 +506,7 @@ The following is a list with the most commonly used process engine configuration
       <br><br>
       A custom hostname can be generated by providing an implementation of the
       {{< javadocref page="org/camunda/bpm/engine/impl/history/event/HostnameProvider.html" text="HostnameProvider" >}} 
-      interface and and setting the engine property `hostnameProvider` to an instance of that class.
+      interface and and setting the engine property <code>hostnameProvider</code> to an instance of that class.
       <br><br>
     </td>
   </tr>
@@ -551,6 +551,20 @@ The following is a list with the most commonly used process engine configuration
       </p>
     </td>
   </tr>
+  <tr>
+    <td><code>transactionIsolationLevel</code></td>
+    <td>String</td>
+    <td>
+        Specifies the value to be configured as the default transaction isolation level for the data source. If <code>null</code> or empty, the value will not be overridden and the global isolation level configured in the database will be used.
+        <p>
+        The default value is <code>readCommitted</code>. If the property is absent, the default value will be set. 
+        </p>
+        <p>
+        <strong>Note</strong>: The <a href="{{<ref "/user-guide/process-engine/database/database-configuration.md#isolation-level-configuration" >}}">recommended transaction isolation level</a> that ensures the correct behaviour of the engine is <code>readCommitted</code>.
+        </p>
+        Values: <code>readCommitted</code>, <code>readUncommitted</code>, <code>repeatableRead</code>, <code>serializable</code> (String).
+    </td>
+  </tr>
   <tr>
     <td><code>jdbcBatchProcessing</code></td>
     <td>Boolean</td>
@@ -1154,7 +1168,7 @@ The following is a list with the most commonly used process engine configuration
     <td>String</td>
     <td><a href="{{< ref "/user-guide/process-engine/history/history-cleanup.md">}}">History cleanup</a> batch window end time in the format <code>HH:mmZ</code> (Z is for RFC 822 time zone) or <code>HH:mm</code>. E.g., <code>23:00-0300</code> or <code>23:00</code>. In case <code>batchWindowEndTime</code> exceeds <code>batchWindowStartTime</code> it is considered
     to be on the same date (e.g., cleanup runs each day between 20:00 and 23:00). Otherwise it is considered to be on the next calendar day (e.g., cleanup starts each
-    day at 20:00 and finishes the next day at 01:00). Default value is `00:00`.</td>
+    day at 20:00 and finishes the next day at 01:00). Default value is <code>00:00</code>.</td>
   </tr>
   <tr>
     <td><code>mondayHistoryCleanupBatchWindowStartTime</code></td>

content/user-guide/process-engine/database/database-configuration.md

@@ -174,3 +174,13 @@ Most database management systems provide four different isolation levels to be s
 * SERIALIZABLE 
 
 The required isolation level to run Camunda with is **READ COMMITTED**, which may have a different name according to your database system. Setting the level to REPEATABLE READS is known to cause deadlocks, so one needs to be careful, when changing the isolation level.
+
+If the isolation level is different from **READ COMMITTED** at the level of the database configuration, the default behavior of the engine is to configure the data source level with this value. 
+
+If you prefer to change this behaviour, you can use property `transactionIsolationLevel` to either use the same value as the global database configuration or use a specific one. This property works as follows:
+
+* If unset, the default value of `readCommitted` will be used to configure the data source.
+* If `null` or empty, the data source won't be configured and the global isolation level for the database will be used.
+* If the property has an accepted value (`readUncommitted`, `readCommitted`, `repeatableReads` or `serializable`), it will be configured in the datasource.
+
+[See here]({{< ref "/reference/deployment-descriptors/tags/process-engine.md#configuration-properties" >}}) for more details about this and other properties.