Update javadoc of FunctionIndex to match the current behaviour

h2/src/main/org/h2/index/FunctionIndex.java

@@ -17,8 +17,8 @@ import org.h2.table.IndexColumn;
 import org.h2.table.TableFilter;
 
 /**
- * An index for a function that returns a result set. This index can only scan
- * through all rows, search is not supported.
+ * An index for a function that returns a result set. Search is this index
+ * performs scan over all rows and should be avoided.
  */
 public class FunctionIndex extends BaseIndex {