Pull request: doc the Question[0] inspection

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

Closes AdguardTeam#2465.

Squashed commit of the following:

commit 0dbcaf7
Merge: 0d7c22c 8a1d86a
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Mon Dec 28 19:09:25 2020 +0300

    Merge branch 'master' into 2465-question-0

commit 0d7c22c
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,

internal/dnsforward/msg.go

@@ -11,7 +11,8 @@ import (
 	"github.com/miekg/dns"
 )
 
-// Create a DNS response by DNS request and set necessary flags
+// makeResponse creates a DNS response by req and sets necessary flags.  It also
+// guarantees that req.Question will be not empty.
 func (s *Server) makeResponse(req *dns.Msg) (resp *dns.Msg) {
 	resp = &dns.Msg{
 		MsgHdr: dns.MsgHdr{