Mark database backend as experimental

We don't guarantee data integrity simple because we don't have full test coverage.
It's only fair to warn users about this.

features.phtml

@@ -161,9 +161,10 @@ include("$top_dir/externals/menu.phtml");
 
 <div class="featurecontainer">
 <div class="featureborder">
-<h2 class="featuretitle"><?php echo T_("Database support");?></h2>
+<h2 class="featuretitle"><?php echo T_("Experimental database support");?></h2>
 </div>
 <p class="justify"><?php echo T_("<span class=\"gnucash\">GnuCash</span> by default stores data in an <b>xml</b> format. Starting with version 2.4, <span class=\"gnucash\">GnuCash</span> financial data can be stored in a <b>SQL database</b> using <b>SQLite3</b>, <b>MySQL</b> or <b>PostgreSQL</b>.");?></p>
+<p class="justify"><?php echo T_("Note this feature is considered experimental.  It works for most of the common use cases but some corner cases have been reported to result in data loss. The <span class=\"gnucash\">GnuCash</span> developers fix each issue as it gets reported. There is however no full test coverage so there may still be scenarios left that result in data loss.");?></p>
 </div>
 
 <div class="featurecontainer">