Adding docstrings to parser.py (#988)

[krisfremen]

Pull Request Checklist

Thank you for taking the time to improve Arrow! Before submitting your pull request, please check all appropriate boxes:

• 🧪 Added tests for changed code.
• 🛠️ All tests pass when run locally (run tox or make test to find out!).
• 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
• 📚 Updated documentation for changed code.
• ⏩ Code is up-to-date with the master branch.

If you have any questions about your code changes or any of the points above, please submit your questions along with the pull request and we will try our best to help!

Description of Changes

Closes #988

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (403c29f) to head (cb00234).
Report is 11 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1010   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         2324      2334   +10     
  Branches       357       357           
=========================================
+ Hits          2324      2334   +10     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[rettinghaus]

These docstrings do not follow PEP 257:

The docstring is a phrase ending in a period. It prescribes the function or method’s effect as a command (“Do this”, “Return that”), not as a description; e.g. don’t write “Returns the pathname …”.