RDKCOM-5534: RDKBDEV-3381 Add xdns Documentation

[Ashrafbhanu-k]

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[SanthoshGujulvajagadeesh]

WAN interface failover and multi-server DNS failover are not provided or handled by XDNS component.

[SanthoshGujulvajagadeesh]

XDNS component provides necessary DML support to configure /etc/resolv.conf which will be consumed by dnsmasq process.