net: indicate in which RFC DnsResourceRecord is found

Give an indication in the documentation where this structure is
defined/specified. So others readers studing this code can read
more information about it.

BUG=None

Change-Id: Iedf747d27231be4982010519365a6caa6dfc1c6d
Reviewed-on: https://chromium-review.googlesource.com/518046
Commit-Queue: Thiago Farina <tfarina@chromium.org>
Reviewed-by: Eric Roman <eroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#475766}

net/dns/dns_response.h

@@ -26,7 +26,8 @@ namespace dns_protocol {
 struct Header;
 }
 
-// Parsed resource record.
+// Structure representing a Resource Record as specified in RFC 1035, Section
+// 4.1.3.
 struct NET_EXPORT_PRIVATE DnsResourceRecord {
   DnsResourceRecord();
   ~DnsResourceRecord();