Pull request: doc the Question[0] inspection

Merge in DNS/adguard-home from 2465-question-0 to master

Closes #2465.

Squashed commit of the following:

commit 0dbcaf7eea4f582cedc31f37d1b20162fe1c38df
Merge: 0d7c22cd0 8a1d86aa7
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Mon Dec 28 19:09:25 2020 +0300

    Merge branch 'master' into 2465-question-0

commit 0d7c22cd0eb930bd301a4bb7b8f23e9fd78a2ccd
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Wed Dec 23 14:17:13 2020 +0300

    dnsforward: doc the Question[0] inspection

internal/dnsforward/dns.go

@@ -41,6 +41,12 @@ func (s *Server) handleDNSRequest(_ *proxy.Proxy, d *proxy.DNSContext) error {
 	ctx.startTime = time.Now()
 
 	type modProcessFunc func(ctx *dnsContext) int
+
+	// Since (*dnsforward.Server).handleDNSRequest(...) is used as
+	// proxy.(Config).RequestHandler, there is no need for additional index
+	// out of range checking in any of the following functions, because the
+	// (*proxy.Proxy).handleDNSRequest method performs it before calling the
+	// appropriate handler.
 	mods := []modProcessFunc{
 		processInitial,
 		processInternalHosts,