[AIRFLOW-XXX] Add missing docs for SNS classes

[sjednac]

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-XXX
  • In case you are fixing a typo in the documentation you can prepend your commit with [AIRFLOW-XXX], code changes always need a Jira issue.

Description

• Here are some details about my PR, including screenshots of any UI changes:

Addresses the missing documentation concern raised by @ashb in #4123.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Just a 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 flake8

[codecov-io]

Codecov Report

Merging #4155 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4155   +/-   ##
=======================================
  Coverage   77.66%   77.66%           
=======================================
  Files         199      199           
  Lines       16273    16273           
=======================================
  Hits        12638    12638           
  Misses       3635     3635

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[kaxil]

Thanks @sbilinski