Source Description returns jinja as a string when trying to render Markdown

[AllenMCunningham]

Issue

When placing markdown into a source description, it renders as the jinja string rather than markdown.

Issue description

The jinja string in the sources description is rendered as the string in the documentation.
Schema.yml file:

markdown file:

rendered description:

Results

The jinja string is returned instead of rendered markdown like in models descriptions
Should render like this:

System information

The output of dbt --version:

installed version: 0.13.1
   latest version: 0.13.1

Up to date!

The operating system you're running on:

Windows 10

The python version you're using (probably the output of python --version)

Python 3.6.8

Steps to reproduce

create an .md doc
dummy markdown provided below:

{% docs tradein %}

### tradein sample
# h1 Heading 8-)
## h2 Heading
### h3 Heading
#### h4 Heading
##### h5 Heading
###### h6 Heading

{% enddocs %}

in schema.yml add a source and reference the doc block created above in the description field

sources:
  - name: Amazon_Tradein
   description: '{{ doc("<name>") }}'

run

dbt docs generate

then run

dbt docs serve

and navigate to the sources description.