**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/www/utils.py
'authorization',
    'api_key',
    'apikey',
    'access_token',
]
def get_sensitive_variables_fields():
    """Get comma-separated sensitive Variable Fields from airflow.cfg."""
    sensitive_fields = set(DEFAULT_SENSITIVE_VARIABLE_FIELDS)
    sensitive_variable_fields = conf.get('admin', 'sensitive_variable_fields')
    if sensitive_variable_fields:
        sensitive_fields.update({field.strip() for field in sensitive_variable_fields.split(',')})
    return sensitive_fields
def should_hide_value_for_key(key_name):
    """Returns True if hide_sensitive_variable_fields is True, else False"""
    
    if key_name:
        config_set = conf.getboolean('admin', 'hide_sensitive_variable_fields')
        field_comp = any(s in key_name.strip().lower() for s in get_sensitive_variables_fields())
        return config_set and field_comp
    return False
def get_params(**kwargs):
    """Return URL-encoded params"""
    return urlencode({d: v for d, v in kwargs.items() if v is not None})
def generate_pages(current_page, num_of_pages, search=None, status=None, window=7):
    """
    Generates the HTML for a paging component using a similar logic to the paging
    auto-generated by Flask managed views. The paging component defines a number of
    pages visible in the pager (window) and once the user goes to a page beyond the
    largest visible, it would scroll to the right the page numbers and keeps the
    current one in the middle of the pager component. When in the last pages,