Clarify ignore patterns in Advanced Usage documentation

[JohnnyWalkerDigital]

Description

Updated the ignore patterns explanation to clarify that they are case-insensitive regular expressions and provided additional details on escaping dots in file extensions.

Related issues/external references

Closes PHPCSStandards/PHP_CodeSniffer#1405

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[github-actions]

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[jrfnl]

Thanks for opening this PR and making those improvement @JohnnyWalkerDigital ! Could you remove the parentheses around the sub-phrase ? Other than that, all good and ready for merge.

[jrfnl]

LGTM Thanks @JohnnyWalkerDigital !