out_datadog: Update the descriptions of special field options

[jszwedko]

These options are useful when there is no attribute in the incoming log that indicates the field to use for the source or service attributes, but otherwise the backend will automatically look for both in the incoming log attributes.

Ref: #8687

Testing Before we can approve your change; please submit the following in a comment:

• [ ] Example configuration file for the change
• [ ] Debug log output from testing the change

• [ ] Attached Valgrind output that shows no leaks or memory corruption was found

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• [ ] Run local packaging test showing all targets (including any new ones) build.
• [ ] Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• [ ] Documentation required for this feature

Backporting

• [ ] Backport to latest stable release.

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[jszwedko]

This is a docs only update so I don't think most of the normal PR checklist is relevant, but let me know if you want me to run any of it.

I'll open a separate PR to update the website docs.

[lecaros]

Hi @jszwedko, thanks for the PR. Please review https://github.com/fluent/fluent-bit/actions/runs/8808240473/job/24176934107?pr=8751

[jszwedko]

Hi @jszwedko, thanks for the PR. Please review https://github.com/fluent/fluent-bit/actions/runs/8808240473/job/24176934107?pr=8751

Thanks for the pointer! I think the PR title should be ok now.

[edsiper]

thanks for the contribution. Note that the requested change by @lecaros was about the commit subject (since this is a minor change I just change it in the squash process, all good)