feat(PSR12.Operators.OperatorSpacing): Add documentation for perCompatible setting

[klausi]

Sister doc PR for PHPCSStandards/PHP_CodeSniffer#1356

PR checklist

• I have checked there is no other PR open for the same change.
• I have read the Contribution Guidelines.
• I grant the project the right to include my changes under the BSD-3-Clause license (and I have the right to grant these rights).
• I have verified that my changes comply with the projects coding standards.

[github-actions]

=== This is an auto-generated comment ===

Thank you for your PR.
A dry-run has been executed on your PR, executing all markdown pre-processing for the wiki files.

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[jrfnl]

Thanks for creating this PR @klausi!

I've left some inline notes.

[klausi]

Thank you as always for the clear guidance and patience, ready to check again!

[jrfnl]

Thanks for the update @klausi ! I will merge this once the upstream PR has been merged.