Update the FormulaEvaluator performance section to match current behaviour

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1348657 13f79535-47bb-0310-9956-ffa450edef68

src/documentation/content/xdocs/spreadsheet/eval.xml

@@ -276,9 +276,22 @@ for(int sheetNum = 0; sheetNum < wb.getNumberOfSheets(); sheetNum++) {
     <section><title>Performance Notes</title>
       <ul>
         <li>Generally you should have to create only one FormulaEvaluator 
-					instance per sheet, but there really is no overhead in creating 
-					multiple FormulaEvaluators per sheet other than that of the 
-					FormulaEvaluator object creation. 
+          instance per Workbook. The FormulaEvaluator will cache 
+          evaluations of dependent cells, so if you have multiple
+          formulas all depending on a cell then subsequent evaluations
+          will be faster.
+        </li>
+        <li>You should normally perform all of your updates to cells,
+          before triggering the evaluation, rather than doing one
+          cell at a time. By waiting until all the updates/sets are
+          performed, you'll be able to take best advantage of the caching
+          for complex formulas.
+        </li>
+        <li>If you do end up making changes to cells part way through
+          evaluation, you should call <em>notifySetFormula</em> or
+          <em>notifyUpdateCell</em> to trigger suitable cache clearance.
+          Alternately, you could instantiate a new FormulaEvaluator,
+          which will start with empty caches.
         </li>
         <li>Also note that FormulaEvaluator maintains a reference to 
           the sheet and workbook, so ensure that the evaluator instance