Add ascii documentation for regexp support in drop_fields processor

[gonzaloe]

What does this PR do?

I've added a description in docs for the regexp functionality from #31424

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[elasticmachine]

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• Start Time: 2022-05-02T17:52:07.790+0000

• Duration: 19 min 55 sec

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[gonzaloe]

@belimawr could you merge the PR? I have no permissions for merging, sadly...