Improve ProgressBar docstring to clarify usage of increments and completion

[QuentinFuxa]

Summary

This pull request refines the documentation for the ProgressBar(Protocol) in _progressbar.py:

1. Clarifies usage of n vs. completed:

   • Emphasizes that n is generally used for page-based increments.
   • Explains that completed is especially useful for percentage-based tracking.

2. Adds a Short Example:

   • Demonstrates a simple implementation showcasing both incremental and absolute completion updates for a plugin

3. Expands the Docstrings:

   • More information about the args used in __init__

Rationale

• Better Guidance: A clearer protocol for developers who need to customize or create their own progress bars.
• Forward Compatibility: Stresses ignoring unrecognized **kwargs so it won’t break on future OCRmyPDF releases.
• Improved Examples: The code snippet helps illustrate how to combine n increments and completed values.

Testing

• Documentation-Only: No production logic is changed, so minimal risk.

Additional Notes

• Please review for style consistency with project standards.
• Suggestions or improvements are always welcome, first PR here!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.51%. Comparing base (6edc749) to head (dda64ce).
Report is 2 commits behind head on main.

Additional details and impacted files

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  Coverage   85.51%   85.51%           
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  Files          95       95           
  Lines        7120     7120           
  Branches      729      729           
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  Hits         6089     6089           
  Misses        818      818           
  Partials      213      213           

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