Update the $req_id placeholder description

[alexbrand]

Updates the documentation for the $req_id placeholder, as it currently does not mention that the value of $req_id is taken from the X-Request-Id header if available, per https://github.com/kubernetes/ingress-nginx/blob/main/rootfs/etc/nginx/template/nginx.tmpl#L396:L403

What this PR does / why we need it:

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation only

Which issue/s this PR fixes

How Has This Been Tested?

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have added tests to cover my changes.
• All new and existing tests passed.

[longwuyuan]

@alexbrand , thanks for your contribution. This PR is good to be merged but we can hopefully make some improvements.

Can you please create a issue and link it here. In the issue, kindly describe the context and background, so new users looking to understand this change can get the info needed. Hopefully you can provide some explanation and some URLs.

/ok-to-test

[iamNoah1]

/kind documentation
/triage accepted
/priority important-soon

@longwuyuan to be honest I don't think we need an Issue in order to get this small change merged.

/lgtm
/hold for @longwuyuan

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: alexbrand, iamNoah1

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[rikatz]

/hold cancel
Thanks!