multilineident: add tests and documentation

[priv-kweihmann]

for the new fix method

Pull request checklist

Bugfix

• [ ] A testcase was added to test the behavior

New feature

• [x] A testcase was added to test the behavior
• [ ] wiki-creator.py was run and a new wiki document was filled with information
• [ ] New functions are documented with docstrings
• [x] No debug code is left
• [x] README.md was updated to reflect the changes (check even if n.a.)