Add REFRESH_HOSTNAMES=UNKNOWN documentation

Signed-off-by: DL6ER <dl6er@dl6er.de>

docs/ftldns/configfile.md

@@ -93,7 +93,7 @@ host name for another IP address (e.g., a DHCP server managed IPv4 address). In
 this case, we use the host name associated to the other address as this is the
 same device.
 
-#### `REFRESH_HOSTNAMES=IPV4|ALL|NONE` (PR [#953](https://github.com/pi-hole/FTL/pull/953)) {#refresh_hostnames data-toc-label='Refresh hostnames'}
+#### `REFRESH_HOSTNAMES=IPV4|ALL|UNKNOWN|NONE` (PR [#953](https://github.com/pi-hole/FTL/pull/953)) {#refresh_hostnames data-toc-label='Refresh hostnames'}
 
 With this option, you can change how (and if) hourly PTR requests are made to check for changes in client and upstream server hostnames. The following options are available:
 
@@ -101,6 +101,8 @@ With this option, you can change how (and if) hourly PTR requests are made to ch
    This is the new default since Pi-hole FTL v5.3.2. It should resolve issues with more and more very short-lived PE IPv6 addresses coming up in a lot of networks.
 - `REFRESH_HOSTNAMES=ALL` - Do the hourly PTR lookups for all addresses
    This is the same as what we're doing with FTL v5.3(.1). This can create a lot of PTR queries for those with many IPv6 addresses in their networks.
+- `REFRESH_HOSTNAMES=UNKNOWN` - Only resolve unknown hostnames
+   Already existing hostnames are never refreshedi, i.e., there will be no PTR queries made for clients where hostnames are known. This also means that known hostnames will not be updated once known.
 - `REFRESH_HOSTNAMES=NONE` - Don't do any hourly PTR lookups
    This means we look host names up exactly once (when we first see a client) and never again. You may miss future changes of host names.