x-pack/filebeat/docs/inputs/http_endpoint: Note that crc.secret is required in http_endpoint docs

[chemamartinez]

What does this PR do?

Clarifies config requirements when using the CRC options.

Why is it important?

Improve UX.

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.

Related issues

• Related to Add CRC validation to Zoom module #35604 (comment)

Use cases

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Logs

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

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• Start Time: 2023-06-06T07:45:15.195+0000

• Duration: 9 min 58 sec

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