**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
""":return: list of outlets defined for this operator"""
        return self._outlets
    @property
    def dag(self) -> Any:
        """Returns the Operator's DAG if set, otherwise raises an error"""
        if self.has_dag():
            return self._dag
        else:
            raise AirflowException(f'Operator {self} has not been assigned to a DAG yet')
    @dag.setter
    def dag(self, dag: Any):
        """
        Operators can be assigned to one DAG, one time. Repeat assignments to
        that same DAG are ok.
        """
        from airflow.models.dag import DAG
        if dag is None:
            self._dag = None
            return
        if not isinstance(dag, DAG):
            raise TypeError(f'Expected DAG; received {dag.__class__.__name__}')
        elif self.has_dag() and self.dag is not dag:
            raise AirflowException(f"The DAG assigned to {self} can not be changed.")
        elif self.task_id not in dag.task_dict:
            dag.add_task(self)
        elif self.task_id in dag.task_dict and dag.task_dict[self.task_id] is not self:
            dag.add_task(self)
        self._dag = dag
    def has_dag(self):