Update helpers.py with DocDB snapshot type

[marty-sullivan]

Issue #, if available:

N/A

Description of changes:

DocumentDB Clusters support DeletionPolicy and UpdateReplacePolicy of type Snapshot

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-attribute-deletionpolicy.html

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.29%. Comparing base (36ff520) to head (f32bb62).
Report is 1 commits behind head on main.

❗ Current head f32bb62 differs from pull request most recent head 5a871ff

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3455      +/-   ##
==========================================
- Coverage   93.30%   93.29%   -0.01%     
==========================================
  Files         324      324              
  Lines       11055    11055              
  Branches     2316     2316              
==========================================
- Hits        10315    10314       -1     
  Misses        415      415              
- Partials      325      326       +1     

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