Update reference documentation for IndexOperations.

Make sure the reference documentation is in line with available methods. Update signatures and remove no longer supported ones.

Closes: #4587

Author: christophstrobl

src/main/asciidoc/reference/mongodb.adoc

@@ -2518,14 +2518,14 @@ The following listing shows the `IndexOperations` interface:
 ----
 public interface IndexOperations {
 
-  void ensureIndex(IndexDefinition indexDefinition);
+  String ensureIndex(IndexDefinition indexDefinition);
+
+  void alterIndex(String name, IndexOptions options);
 
   void dropIndex(String name);
 
   void dropAllIndexes();
 
-  void resetIndexCache();
-
   List<IndexInfo> getIndexInfo();
 }
 ----