Improve docs

[forman]

Improve docs

Checklist (strike out non-applicable):

• Changes documented in CHANGES.md
• Related issue exists and is referred to in the PR description and CHANGES.md
• Added docstrings and API docs for any new/modified user-facing classes and functions
• Changes/features documented in docs/*
• Unit-tests adapted/added for changes/features
• Test coverage remains or increases (target 100%)

[codecov]

Codecov Report

Attention: Patch coverage is 72.00000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 96.17%. Comparing base (de5c4e5) to head (e6bbaac).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
xrlint/processor.py	40.00%	6 Missing ⚠️
xrlint/rule.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #18      +/-   ##
==========================================
- Coverage   96.26%   96.17%   -0.09%     
==========================================
  Files          51       51              
  Lines        2061     2065       +4     
  Branches      269      270       +1     
==========================================
+ Hits         1984     1986       +2     
- Misses         44       46       +2     
  Partials       33       33              

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