Added documentation for the record expiration feature

Author: peteeckel

docs/using_netbox_dns.md

@@ -1172,6 +1172,29 @@ Another example shows a conflict at View level when assigning a prefix:
 
 The solution is to either disable automatic DNS record configuration for the affected IP addresses (which retains the original DNS records) or to remove or deactivate the existing address records and try again.
 
+### Record Expiration
+
+In some cases it may be desired to define an expiration date for records, for example for validation `TXT` records that are only necessary for a limited period of time. Starting with NetBox DNS 1.5.10, a feature was added that enables automatic expiration of records.
+
+When the "Expiration Date" is set for a record, the record is supposed to be deactivated when this date is reached. For this automatism to be effective, two conditions need to be met:
+
+* The record expiration job needs to be enabled in NetBox DNS
+* The "Generate SOA SERIAL" feature must be active for the zone (which is the default setting)
+
+The record expiration job is disabled by default because it adds some overhead to NetBox and is not necessary unless the record expiration feature is to be used. To enable it, the `enable_record_exiration_job` variable must be set in the NetBox DNS configuration:
+
+```
+PLUGINS_CONFIG = {
+    'netbox_dns': {
+        ...
+        'enable_record_expiration_job': True,
+        ...
+    },
+}
+```
+
+If enabled, the job runs once daily and sets the status of all records whose expiration date has passed to "Expired". This is an inactive record status and causes the record to be disregarded by provisioning mechanisms.
+
 ### Additional Information for IP Addresses and DNS Records
 When a link between an IP address and a DNS address record is present, additional panes will appear in the IPAM IP address and NetBox DNS record view, as well as in the detail views for NetBox DNS managed records.
 
@@ -1213,7 +1236,6 @@ For records automatically generated by DNSsync, this is not generally the desire
 
 For this reason, NetBox DNS does not check uniqueness for address records automatically generated by DNSsync, and there is an option to deactivate other conflicting address records automatically. This is not enabled by default, but it can be switched on with the configuration variable `dnssync_conflict_deactivate`:
 
-
 ```
 PLUGINS_CONFIG = {
     'netbox_dns': {