Pull request: client: imp private dns resolver docs

Updates #3028.

Squashed commit of the following:

commit a44f1b7
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Apr 29 17:36:20 2021 +0300

    client: imp private dns resolver docs

client/src/__locales/en.json

@@ -9,7 +9,7 @@
     "bootstrap_dns": "Bootstrap DNS servers",
     "bootstrap_dns_desc": "Bootstrap DNS servers are used to resolve IP addresses of the DoH/DoT resolvers you specify as upstreams.",
     "local_ptr_title": "Private DNS servers",
-    "local_ptr_desc": "The DNS servers that AdGuard Home will use for queries for locally served resources. For instance, this server will be used for resolving clients' hostnames for the clients with private IP addresses. If not set, AdGuard Home will automatically use your default DNS resolver.",
+    "local_ptr_desc": "The DNS servers that AdGuard Home uses for local PTR queries. These servers are used to resolve the hostnames of clients with private IP addresses, for example \"192.168.12.34\", using rDNS. If not set, AdGuard Home uses the default DNS resolvers of your OS.",
     "local_ptr_placeholder": "Enter one server address per line",
     "resolve_clients_title": "Enable reverse resolving of clients' IP addresses",
     "resolve_clients_desc": "If enabled, AdGuard Home will attempt to reversely resolve clients' IP addresses into their hostnames by sending PTR queries to corresponding resolvers (private DNS servers for local clients, upstream server for clients with public IP addresses).",