refactor: Enhance docs, add more tests in `IntegerToEnglish`

[Hardvan]

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized it.
• [x] All filenames are in PascalCase.
• [x] All functions and variable names follow Java naming conventions.
• [x] All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• [x] All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

:x: Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review. :white_check_mark: Project coverage is 67.24%. Comparing base (2f9f75a) to head (0483791). :warning: Report is 388 commits behind head on master.

Files with missing lines	Patch %	Lines
...om/thealgorithms/conversions/IntegerToEnglish.java	94.44%	0 Missing and 1 partial :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5924      +/-   ##
============================================
+ Coverage     67.18%   67.24%   +0.05%     
- Complexity     4561     4566       +5     
============================================
  Files           620      620              
  Lines         17142    17149       +7     
  Branches       3310     3311       +1     
============================================
+ Hits          11517    11531      +14     
+ Misses         5167     5164       -3     
+ Partials        458      454       -4     

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