[master] Add comment and change log entry for 4292

ChangeLog

+1085.	[bug]		sar
+	Always copy the DHCPv4 flags field from a client's request to the
+	the server's resposne.
+	(Trac #4292, git <tbd>)
+
 1084.	[bug]		fdupont
 	Removed obsolete requirement of python3 by --enable-generate-docs
 	configuration option.

src/bin/dhcp4/dhcp4_srv.cc

@@ -199,6 +199,7 @@ Dhcpv4Exchange::copyDefaultFields() {
         resp_->setRemoteHWAddr(dst_hw_addr);
     }
 
+    // Copy flags from the request to the response per RFC 2131
     resp_->setFlags(query_->getFlags());
 }