Repository: dbt-labs/dbt-core

Issue:
Source Description returns jinja as a string when trying to render Markdown
## 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:
![image](https://user-images.githubusercontent.com/43586980/58519232-7d945b00-8167-11e9-87ee-06598d2ea4b4.png)

markdown file:
![image](https://user-images.githubusercontent.com/43586980/58519323-e54aa600-8167-11e9-8fa7-083a9cd086ad.png)

rendered description:
![image](https://user-images.githubusercontent.com/43586980/58519340-ff848400-8167-11e9-90d5-75247ab73cdc.png)


### Results
The jinja string is returned instead of rendered markdown like in models descriptions
Should render like this: 
![image](https://user-images.githubusercontent.com/43586980/58519524-ddd7cc80-8168-11e9-851a-c4eeac473093.png)

### 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.


Discussion:

Resolving patch:
diff --git a/core/dbt/parser/util.py b/core/dbt/parser/util.py
--- a/core/dbt/parser/util.py
+++ b/core/dbt/parser/util.py
@@ -1,6 +1,7 @@
 
 import dbt.exceptions
 import dbt.utils
+from dbt.node_types import NodeType
 
 
 def docs(node, manifest, config, column_name=None):
@@ -146,10 +147,27 @@ def process_docs_for_node(cls, manifest, current_project, node):
             else:
                 column['description'] = description
 
+    @classmethod
+    def process_docs_for_source(cls, manifest, current_project, source):
+        context = {
+            'doc': docs(source, manifest, current_project),
+        }
+        table_description = source.get('description', '')
+        source_description = source.get('source_description', '')
+        table_description = dbt.clients.jinja.get_rendered(table_description,
+                                                           context)
+        source_description = dbt.clients.jinja.get_rendered(source_description,
+                                                            context)
+        source.set('description', table_description)
+        source.set('source_description', source_description)
+
     @classmethod
     def process_docs(cls, manifest, current_project):
         for node in manifest.nodes.values():
-            cls.process_docs_for_node(manifest, current_project, node)
+            if node.resource_type == NodeType.Source:
+                cls.process_docs_for_source(manifest, current_project, node)
+            else:
+                cls.process_docs_for_node(manifest, current_project, node)
         return manifest
 
     @classmethod


Test patch:
diff --git a/test/integration/029_docs_generate_tests/test_docs_generate.py b/test/integration/029_docs_generate_tests/test_docs_generate.py
--- a/test/integration/029_docs_generate_tests/test_docs_generate.py
+++ b/test/integration/029_docs_generate_tests/test_docs_generate.py
@@ -1266,7 +1266,7 @@ def expected_postgres_references_manifest(self, model_database=None):
                     'resource_type': 'source',
                     'root_path': os.getcwd(),
                     'schema': my_schema_name,
-                    'source_description': "{{ doc('source_info') }}",
+                    'source_description': 'My source',
                     'source_name': 'my_source',
                     'unique_id': 'source.test.my_source.my_table'
                 }