reverse-proxy: T6370: Documented usage of http-response-headers option (backport #1463)

[mergify]

Change Summary

Made changes to reverse-proxy.rst for the following:

• Documented usage of http-response-headers option

Related Task(s)

• https://vyos.dev/T6370

Related PR(s)

• reverse-proxy: T6370: Set custom HTTP headers in reverse-proxy responses vyos-1x#3487

Backport

Checklist:

• I have read the CONTRIBUTING document

  ---

  This is an automatic backport of pull request reverse-proxy: T6370: Documented usage of http-response-headers option #1463 done by Mergify.

[mergify]

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	both modified:   docs/configuration/loadbalancing/reverse-proxy.rst

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