DATAMONGO-1805 - Update GridFsOperations documentation.

Fix return type in reference documentation and update Javadoc.

src/main/asciidoc/reference/mongodb.adoc

@@ -2602,7 +2602,7 @@ class GridFsClient {
 
   @Test
   public void findFilesInGridFs() {
-    List<GridFSDBFile> result = operations.find(query(whereFilename().is("filename.txt")))
+    GridFSFindIterable result = operations.find(query(whereFilename().is("filename.txt")))
   }
 }
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