**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.__dict__ = state  
        self._log = logging.getLogger("airflow.task.operators")
    def render_template_fields(self, context: Dict, jinja_env: Optional[jinja2.Environment] = None) -> None:
        """
        Template all attributes listed in template_fields. Note this operation is irreversible.
        :param context: Dict with values to apply on content
        :type context: dict
        :param jinja_env: Jinja environment
        :type jinja_env: jinja2.Environment
        """
        if not jinja_env:
            jinja_env = self.get_template_env()
        self._do_render_template_fields(self, self.template_fields, context, jinja_env, set())
    def _do_render_template_fields(
        self,
        parent: Any,
        template_fields: Iterable[str],
        context: Dict,
        jinja_env: jinja2.Environment,
        seen_oids: Set,
    ) -> None:
        for attr_name in template_fields:
            content = getattr(parent, attr_name)
            if content:
                rendered_content = self.render_template(content, context, jinja_env, seen_oids)
                setattr(parent, attr_name, rendered_content)
    def render_template(