added Alternative Domain section to dns page in docs

website/content/docs/discovery/dns.mdx

@@ -96,6 +96,33 @@ pairs according to [RFC1464](https://www.ietf.org/rfc/rfc1464.txt).
 Alternatively, the TXT record will only include the node's metadata value when the
 node's metadata key starts with `rfc1035-`.
 
+## Alternative Domain
+
+Consul support setting an alternative domain, which is usefull during DNS migrations or if you want to respond to
+internal and external queries with different domains.
+
+If you are using [`alt_domain`](/docs/agent/options#alt_domain) parameter, Consul will respond  based on the presence of it in your queries.
+For exmaple if `test-domain` is configured as an alternative domain the following query:
+
+```shell-session
+$ dig @127.0.0.1 -p 8600  consul.service.test-domain SRV
+```
+will return:
+
+```
+;; QUESTION SECTION:
+;consul.service.test-domain.	IN	SRV
+
+;; ANSWER SECTION:
+consul.service.test-domain. 0	IN	SRV	1 1 8300 machine.node.dc1.test-domain.
+
+;; ADDITIONAL SECTION:
+machine.node.dc1.test-domain. 0	IN	A	127.0.0.1
+machine.node.dc1.test-domain. 0	IN	TXT	"consul-network-segment="
+```
+
+-> **Note:** Response to `<ip>.in-addr.arpa.` will always be returned with your default domain, as there is no way to identify queried domain.
+
 ## Service Lookups
 
 A service lookup is used to query for service providers. Service queries support