Added more documentation.

docs/add_rtadv.md

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+# `add_rtadv` Command
+
+## Name
+
+`add_rtadv`
+
+## Synopsis
+
+```
+add_rtadv ${card} ${vlan}
+```
+
+## Description
+
+Adds the network defined by the specified `${card}` and `${vlan}` to the list of
+interfaces to be advertised by `rtadvd`.  
+
+The internal implementation prevents adding duplicate interfaces to the
+advertisement list, by checking for their presence before adding.
+It can safely be used alongside manual changes to `rc.conf` changes to
+the `rtadvd_interfaces` variable.
+
+Additionally, the `${rc_router_dir}/rc.d/generate_rtadvd_conf` service script
+will generate `${rc_router_gen_dir}/rtadvd.conf` with `${interface}:tc=settings`
+lines therein which can be used by the `rtadvd` that comes with FreeBSD.
+Various `rc.router` [variables](variables.md) can be used to control the global
+settings for all advertised IPv6 networks.
+
+## Examples
+
+```
+add_rtadv xy0 123 # Adds `xy0.123` to `rtadvd_interfaces`
+add_rtadv abc3 765 # Adds `abc3.765` to `rtadvd_interfaces`
+```
+
+## See Also
+ - [`add_net`](add_net.md)
+ - [`add_rtadv`](add_rtadv.md)