[AIRFLOW-5702] Fix common docstring issues

[mik-laj]

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-5702
  • 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.
  • In case you are proposing a fundamental code change, you need to create an Airflow Improvement Proposal (AIP).
  • In case you are adding a dependency, check if the license complies with the ASF 3rd Party License Policy.

Description

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

Tests

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

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.
  • All the public functions and the classes in the PR contain docstrings that explain what it does
  • If you implement backwards incompatible changes, please leave a note in the Updating.md so we can assign it to a appropriate release

[feluelle]

A minor issue: /opt/airflow/docs/_api/airflow/hooks/base_hook/index.rst:56: WARNING: Field list ends without a blank line; unexpected unindent.

[codecov-io]

Codecov Report

Merging #6372 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6372      +/-   ##
==========================================
+ Coverage   80.39%   80.41%   +0.01%     
==========================================
  Files         616      616              
  Lines       35804    35804              
==========================================
+ Hits        28783    28790       +7     
+ Misses       7021     7014       -7

Impacted Files	Coverage Δ
airflow/configuration.py	89.13% <ø> (ø)	⬆️
airflow/models/baseoperator.py	95.62% <ø> (ø)	⬆️
airflow/www/utils.py	75.6% <ø> (ø)	⬆️
airflow/bin/cli.py	66.21% <ø> (ø)	⬆️
airflow/gcp/hooks/dataproc.py	55.25% <ø> (ø)	⬆️
airflow/ti_deps/deps/dagrun_id_dep.py	100% <ø> (ø)	⬆️
airflow/ti_deps/deps/pool_slots_available_dep.py	100% <ø> (ø)	⬆️
airflow/executors/local_executor.py	82.17% <ø> (ø)	⬆️
airflow/contrib/hooks/aws_firehose_hook.py	100% <ø> (ø)	⬆️
airflow/contrib/hooks/grpc_hook.py	91.93% <ø> (ø)	⬆️
... and 35 more

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