[AIRFLOW-1997] Fix doc strings of GCP operators

[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-1997

Description

• Here are some details about my PR, including screenshots of any UI changes:
  Fixed doc strings from DataProc and GCS Operators so that would aid in automatic documentation generation.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:
  It just changes the docstrings. Hence no tests required.

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
  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"

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

[codecov-io]

Codecov Report

Merging #2939 into master will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2939      +/-   ##
==========================================
- Coverage   73.54%   73.47%   -0.07%     
==========================================
  Files         160      160              
  Lines       12208    12208              
==========================================
- Hits         8978     8970       -8     
- Misses       3230     3238       +8

Impacted Files	Coverage Δ
airflow/task/task_runner/bash_task_runner.py	93.33% <0%> (-6.67%)	⬇️
airflow/utils/helpers.py	53.44% <0%> (-2.88%)	⬇️
airflow/jobs.py	79.48% <0%> (-0.27%)	⬇️
airflow/models.py	87% <0%> (+0.04%)	⬆️

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[criccomini]

LGTM