[AIRFLOW-2864] Fix docstrings for SubDagOperator

[kaxil]

Make sure you have checked all steps below.

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-2864

Description

• Here are some details about my PR, including screenshots of any UI changes:
  The docstrings are currently in the __init__ method, due to which they are not automatically shown in the Sphinx documentation.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:
  N/a - not needed. Just doc change

Commits

• My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.

Code Quality

• Passes git diff upstream/master -u -- "*.py" | flake8 --diff

[ashb]

Curious. I thought sphinx looked here for class-level docs. I guess not!

[kaxil]

I think it has something to do with configurations:

• https://stackoverflow.com/questions/5599254/how-to-use-sphinxs-autodoc-to-document-a-classs-init-self-method
• http://www.sphinx-doc.org/en/stable/ext/example_google.html

[codecov-io]

Codecov Report

Merging #3712 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3712   +/-   ##
=======================================
  Coverage   77.56%   77.56%           
=======================================
  Files         204      204           
  Lines       15766    15766           
=======================================
  Hits        12229    12229           
  Misses       3537     3537

Impacted Files	Coverage Δ
airflow/operators/subdag_operator.py	90.32% <ø> (ø)	⬆️

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