📝 add note on parsing ordered_json #3325 (#3326)

doc/mkdocs/docs/features/object_order.md

@@ -2,6 +2,8 @@
 
 The [JSON standard](https://tools.ietf.org/html/rfc8259.html) defines objects as "an unordered collection of zero or more name/value pairs". As such, an implementation does not need to preserve any specific order of object keys.
 
+## Default behavior: sort keys
+
 The default type `nlohmann::json` uses a `std::map` to store JSON objects, and thus stores object keys **sorted alphabetically**.
 
 ??? example
@@ -33,6 +35,8 @@ The default type `nlohmann::json` uses a `std::map` to store JSON objects, and t
     }
     ```
 
+## Alternative behavior: preserve insertion order
+
 If you do want to preserve the **insertion order**, you can try the type [`nlohmann::ordered_json`](https://github.com/nlohmann/json/issues/2179).
 
 ??? example
@@ -65,3 +69,58 @@ If you do want to preserve the **insertion order**, you can try the type [`nlohm
     ```
 
 Alternatively, you can use a more sophisticated ordered map like [`tsl::ordered_map`](https://github.com/Tessil/ordered-map) ([integration](https://github.com/nlohmann/json/issues/546#issuecomment-304447518)) or [`nlohmann::fifo_map`](https://github.com/nlohmann/fifo_map) ([integration](https://github.com/nlohmann/json/issues/485#issuecomment-333652309)).
+
+### Notes on parsing
+
+Note that you also need to call the right [`parse`](../api/basic_json/parse.md) function when reading from a file.
+Assume file `input.json` contains the JSON object above:
+
+```json
+{
+  "one": 1,
+  "two": 2,
+  "three": 3
+}
+```
+
+!!! success "Right way"
+
+    The following code correctly calls the `parse` function from `nlohmann::ordered_json`:
+
+    ```cpp
+    std::ifstream i("input.json");
+    auto j = nlohmann::ordered_json::parse(i);
+    std::cout << j.dump(2) << std::endl;
+    ```
+
+    The output will be:
+
+    ```json
+    {
+      "one": 1,
+      "two": 2,
+      "three": 3
+    }
+    ```
+
+??? failure "Wrong way"
+
+    The following code incorrectly calls the `parse` function from `nlohmann::json` which does not preserve the
+    insertion order, but sorts object keys. Assigning the result to `nlohmann::ordered_json` compiles, but does not
+    restore the order from the input file.
+
+    ```cpp
+    std::ifstream i("input.json");
+    nlohmann::ordered_json j = nlohmann::json::parse(i);
+    std::cout << j.dump(2) << std::endl;
+    ```
+
+    The output will be:
+
+    ```json
+    {
+      "one": 1,
+      "three": 3
+      "two": 2,
+    }
+    ```