**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
"""Returns dictionary of all global extra links"""
        from airflow import plugins_manager
        plugins_manager.initialize_extra_operators_links_plugins()
        if plugins_manager.global_operator_extra_links is None:
            raise AirflowException("Can't load operators")
        return {link.name: link for link in plugins_manager.global_operator_extra_links}
    @prepare_lineage
    def pre_execute(self, context: Any):
        """This hook is triggered right before self.execute() is called."""
    def execute(self, context: Any):
        """
        This is the main method to derive when creating an operator.
        Context is the same dictionary used as when rendering jinja templates.
        Refer to get_template_context for more context.
        """
        raise NotImplementedError()
    @apply_lineage
    def post_execute(self, context: Any, result: Any = None):
        """
        This hook is triggered right after self.execute() is called.
        It is passed the execution context and any results returned by the
        operator.
        """
    def on_kill(self) -> None: