Align docs with PLAIN matching

[phoenikz]

Documentation states that PLAIN matches with case sensitivity, but the implementation is with case insensitivity. This aligns documentation with implementation

Mutually exclusive with #474, proposing that maintainers select one and reject one - I have no opinion how it should work, just that is should be consistent

• [x ] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• [ x] Ensure that the pull request title represents the desired changelog entry
• [ x] Please describe what you did
• [ x] Link to relevant issues in GitHub or Jira
• [ x] Link to relevant pull requests, esp. upstream and downstream changes
• [ x] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[rsandell]

Thanks!