Fix description for bulk write update operation

Author: isabelatkinson

source/crud/bulk-write.md

@@ -518,10 +518,15 @@ and add it as the `_id` field at the beginning of the document.
     "multi": Optional<Boolean>,
     "upsert": Optional<Boolean>,
     "arrayFilters": Optional<Array>,
-    "hint": Optional<Document | String>
+    "hint": Optional<Document | String>,
+    "collation": Optional<Document>
 }
 ```
 
+The `update` command document is used for update and replace operations. For update operations, the `updateMods` field
+corresponds to the `update` field in `UpdateOneModel` and `UpdateManyModel`. For replace operations, the `updateMods`
+field corresponds to the `replacement` field in `ReplaceOneModel`.
+
 #### Delete
 
 ```javascript