document how decode_xml_wineventlog maps fields

[leehinman]

What does this PR do?

Documents the field mappings for the decode_xml_wineventlog processor.

Why is it important?

Needed so users will know what fields will be populated so they can filter on them in Conditionals or Logstash.

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.

Screenshots

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b decode_xml_wineventlog_docs upstream/decode_xml_wineventlog_docs
git merge upstream/main
git push upstream decode_xml_wineventlog_docs

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[elasticmachine]

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• Start Time: 2022-07-22T16:08:35.602+0000

• Duration: 20 min 12 sec

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[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b decode_xml_wineventlog_docs upstream/decode_xml_wineventlog_docs
git merge upstream/main
git push upstream decode_xml_wineventlog_docs