Docs blocks don't work in exposures - 'doc' is undefined

[joellabes]

Describe the bug

Exposures containing a docs block don't compile in dbt Cloud because doc is undefined.

Steps To Reproduce

Create an exposure which references a doc block:

exposures:
  - name: licence_status_stats
    description: '{{ doc("licence_status_stats_descr") }}'
    type: dashboard
...

{% docs licence_status_stats_descr %}

A high-level answer to the question: _How many students/departments/schools/countries use EP?_
----------------------------------------------------------------------------------------------

You can find the definitions of each licence status [here](https://sites.google.com/educationperfect.com/epeeps-hub/must-read/data-definitions). 

{% enddocs %}

Expected behavior

The docs block to be rendered

Screenshots and log output

Compilation Error Failed to render models/exposures/licence_status_stats.yml from project educationperfect: Compilation Error Could not render {{ doc("licence_status_stats_descr") }}: 'doc' is undefined

System information

Which database are you using dbt with?

• postgres
• redshift
• bigquery
• snowflake
• other (specify: ____________)

The output of dbt --version:

0.18.1

The operating system you're using:
dbt Cloud
The output of python --version:

Additional context

Add any other context about the problem here.

[jtcohen6]

Nice find @joellabes! This was definitely an oversight for v0.18.1; I imagine the fix to be quite straightforward, and I'd love to get it into v0.19.0.

In particular, I think a prospective contributor may find it fruitful to:

1. Add exposures to the list of documentable resources:
   https://github.com/fishtown-analytics/dbt/blob/fec0e31a25d5b922cb1833cffcb5095eb4ee642b/core/dbt/node_types.py#L40-L49

2. Add exposures as a step within process_docs, after first defining _process_docs_for_exposure, with the existing methods as a helpful guide:
   https://github.com/fishtown-analytics/dbt/blob/fec0e31a25d5b922cb1833cffcb5095eb4ee642b/core/dbt/parser/manifest.py#L625-L649

3. With the prior two pieces in place, when the contributor goes to test their docs-block-using exposure, they will stumble upon a new error message like: Invalid exposures config... Additional properties are not allowed ('original_file_path', 'package_name', 'yaml_key' were unexpected). After some initial trepidation, they may be delighted to learn that they need change only one more line of code:

class UnparsedExposure(HasYamlMetadata, Replaceable):

That's just some wild speculation, though :)