REFACTOR: Improve assign balloon method

[SMoraisAnsys]

Description

Little follow up to #6011 (comment)
On top of updating the docstring (content and typo), I added a warning when the option is used for non Electrostatic solution type.

Issue linked

Associated to #5991

Checklist

• I have tested my changes locally.
• I have added necessary documentation or updated existing documentation.
• I have followed the coding style guidelines of this project.
• I have added appropriate tests (unit, integration, system).
• I have reviewed my changes before submitting this pull request.
• I have linked the issue or issues that are solved by the PR if any.
• I have agreed with the Contributor License Agreement (CLA).

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.06%. Comparing base (d1c3dc3) to head (fdf00b3).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6017      +/-   ##
==========================================
+ Coverage   82.35%   85.06%   +2.70%     
==========================================
  Files         166      166              
  Lines       62980    62981       +1     
==========================================
+ Hits        51869    53574    +1705     
+ Misses      11111     9407    -1704     

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