Update Unbound->Overrides documentation

[swhite2]

Important notices

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• [x] I have read the contributing guide lines at https://github.com/opnsense/docs/blob/master/CONTRIBUTING.md
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Unbound -> Overrides is changing a bit so it would make sense to keep this up-to-date in the documentation. It isn't very well documented in general so an elaboration here is fitting.

Things to consider:

• How PTR records are generated.
• Clearly describe how system A/AAAA records can be manipulated.
• Hint that domain overrides shouldn't really be used for it's functionality as it functions completely different from a Host Override, instead refer to the Query Forwarding page.

[swhite2]

Done in https://github.com/opnsense/docs/commit/a67e130d238b89f863ab2f7ef83e7253d0ad1412.

@fichtner Parking it here until the mentioned items are in release:

• DNSSEC + rebinding update: https://github.com/opnsense/core/commit/f3efe39f7b7e182bb77a7dd3a371fe4c5c054585
• A/AAAA/PTR records: https://github.com/opnsense/core/commit/92a5a224dd3d08df822cbe722ba5657db1ac7991

[fichtner]

Just for completeness from roadmap discussion: overrides and query forwarding are the same thing according to the forum https://forum.opnsense.org/index.php?topic=29813.0

[swhite2]

This was completed with https://github.com/opnsense/docs/commit/1b888f7233bfde1b96411abbc470196a09fde21f