**Apache Airflow version**: 1.10.14

**Environment**:

- **Cloud provider or hardware configuration**: docker
- **OS** (e.g. from /etc/os-release): apache/airflow:1.10.14-python3.8
- **Kernel** (e.g. `uname -a`): Linux host 5.4.0-62-generic #70-Ubuntu SMP Tue Jan 12 12:45:47 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
- **Install tools**: Docker version 19.03.8, build afacb8b7f0
- **Others**:

**What happened**:
I created a DAG and set the doc_md property on the object but it isn't being rendered in the UI.

**What you expected to happen**:
I expected the markdown to be rendered in the UI

**How to reproduce it**:
Created a new container using the `airflow:1.10.14`, I have tried the following images with the same results.

-  airflow:1.10.14:image-python3.8
-  airflow:1.10.14:image-python3.7
-  airflow:1.10.12:image-python3.7
-  airflow:1.10.12:image-python3.7

```
dag_docs = """
## Pipeline

#### Purpose
This is a pipeline
"""

dag = DAG(
    'etl-get_from_api',
    default_args=default_args,
    description='A simple dag',
    schedule_interval=timedelta(days=1),
)
dag.doc_md = dag_docs

```

![image](https://user-images.githubusercontent.com/29732449/105004686-6b77d680-5a88-11eb-9e34-c8dd38b3fd10.png)

![image](https://user-images.githubusercontent.com/29732449/105004748-7af71f80-5a88-11eb-811c-11bc6a351c71.png)

I have also tried with using a doc-string to populate the doc_md as well as adding some text within the constructor.

```
dag = DAG(
    'etl-get_from_api',
    default_args=default_args,
    description='A simple dag',
    schedule_interval=timedelta(days=1),
    doc_md = "some text"
)
```
All of the different permutations I've tried seem to have the same result. The only thing I can change is the description, that appears to show up correctly.

**Anything else we need to know**:
I have tried multiple browsers (Firefox and Chrome) and I have also done an inspect on from both the graph view and the tree view from within the dag but I can't find any of the text within the page at all.

tests/serialization/test_dag_serialization.py
"ui_color": "#f0ede4",
                "ui_fgcolor": "#000",
                "template_fields": ['bash_command', 'env'],
                "template_fields_renderers": {'bash_command': 'bash', 'env': 'json'},
                "bash_command": "echo {{ task.task_id }}",
                'label': 'bash_task',
                "_task_type": "BashOperator",
                "_task_module": "airflow.operators.bash",
                "pool": "default_pool",
                "executor_config": {
                    '__type': 'dict',
                    '__var': {
                        "pod_override": {
                            '__type': 'k8s.V1Pod',
                            '__var': PodGenerator.serialize_pod(executor_config_pod),
                        }
                    },
                },
            },
            {
                "task_id": "custom_task",
                "retries": 1,
                "retry_delay": 300.0,
                "max_retry_delay": 600.0,
                "sla": 100.0,
                "_downstream_task_ids": [],
                "_inlets": [],
                "_is_dummy": False,
                "_outlets": [],
                "_operator_extra_links": [{"tests.test_utils.mock_operators.CustomOpLink": {}}],