**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.

airflow/models/baseoperator.py
self.max_retry_delay = max_retry_delay
            else:
                self.log.debug("Max_retry_delay isn't timedelta object, assuming secs")
                self.max_retry_delay = timedelta(seconds=max_retry_delay)  
        self.params = params or {}  
        self.priority_weight = priority_weight
        if not WeightRule.is_valid(weight_rule):
            raise AirflowException(
                "The weight_rule must be one of {all_weight_rules},"
                "'{d}.{t}'; received '{tr}'.".format(
                    all_weight_rules=WeightRule.all_weight_rules,
                    d=dag.dag_id if dag else "",
                    t=task_id,
                    tr=weight_rule,
                )
            )
        self.weight_rule = weight_rule
        self.resources: Optional[Resources] = Resources(**resources) if resources else None
        self.run_as_user = run_as_user
        self.task_concurrency = task_concurrency
        self.executor_config = executor_config or {}
        self.do_xcom_push = do_xcom_push
        
        self._upstream_task_ids: Set[str] = set()
        self._downstream_task_ids: Set[str] = set()
        self._dag = None
        self.dag = dag or DagContext.get_current_dag()
        
        
        from airflow.models.dag import DAG