Improve the documentation of "Router Advertisements"

[szotsaki]

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Describe the bug

Router advertisements documentation is missing all the advanced options.

For example neither "AdvRemoveRoute", nor "AdvDeprecatePrefix" is documented in the radvd man page either for Linux or for FreeBSD.

What's the difference between these two?

To Reproduce

Steps to reproduce the behavior:

1. After enabling manual RA handling, go to Services > Router Advertisement > Advanced
2. Try to find docs related to the options

Environment

Software version used and hardware type if relevant, e.g.:

OPNsense 25.1

[fichtner]

AdvRemoveRoute and AdvDeprecatePrefix are advanced overrides of the RemoveRoute and DeprecatePrefix radvd.conf flags when users think these values automatically adjusted by the GUI are wrong (this includes a toggle for CARP aware setup for HA). These values are NOT for general consumption and will be removed eventually. Their code intention is clear and direct:

$radvdconf .= "\t\tDeprecatePrefix " . (!empty($dhcpv6ifconf['AdvDeprecatePrefix']) ? $dhcpv6ifconf['AdvDeprecatePrefix'] : ($carp_mode ? 'off' : 'on')) . ";\n";

and

$radvdconf .= "\t\tRemoveRoute " . (!empty($dhcpv6ifconf['AdvRemoveRoute']) ? $dhcpv6ifconf['AdvRemoveRoute'] : ($carp_mode ? 'off' : 'on')) . ";\n";