**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
of this task fails. a context dictionary is passed as a single
        parameter to this function. Context contains references to related
        objects to the task instance and is documented under the macros
        section of the API.
    :type on_failure_callback: TaskStateChangeCallback
    :param on_execute_callback: much like the ``on_failure_callback`` except
        that it is executed right before the task is executed.
    :type on_execute_callback: TaskStateChangeCallback
    :param on_retry_callback: much like the ``on_failure_callback`` except
        that it is executed when retries occur.
    :type on_retry_callback: TaskStateChangeCallback
    :param on_success_callback: much like the ``on_failure_callback`` except
        that it is executed when the task succeeds.
    :type on_success_callback: TaskStateChangeCallback
    :param trigger_rule: defines the rule by which dependencies are applied
        for the task to get triggered. Options are:
        ``{ all_success | all_failed | all_done | one_success |
        one_failed | none_failed | none_failed_or_skipped | none_skipped | dummy}``
        default is ``all_success``. Options can be set as string or
        using the constants defined in the static class
        ``airflow.utils.TriggerRule``
    :type trigger_rule: str
    :param resources: A map of resource parameter names (the argument names of the
        Resources constructor) to their values.
    :type resources: dict
    :param run_as_user: unix username to impersonate while running the task
    :type run_as_user: str
    :param task_concurrency: When set, a task will be able to limit the concurrent
        runs across execution_dates
    :type task_concurrency: int