docs(config): clarify forwarded_for_headers configuration

[joshtrichards]

• Resolves: #

Summary

• Correct the forwarded_for_headers example to use HTTP_X_FORWARDED_FOR and remove the confusing references to HTTP_X_FORWARDED and HTTP_FORWARDED_FOR
• Note that the default HTTP_X_FORWARDED_FOR configuration is suitable for many environments and may not need to be changed
• Document the trusted-proxy security requirements and comma-separated address handling
• Link to the X-Forwarded-For header reference

Came up due to some confusion on the forum, since a quick scan of the entry could lead one to believe the examples are valid headers/header keys (they aren't): https://help.nextcloud.com/t/administration-settings-not-showing-via-an-https-connection/248505

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[joshtrichards]

/backport to stable34

[joshtrichards]

/backport to stable33