Doxygen: add several missing links.

src/feature/dirauth/feature_dirauth.dox

@@ -3,4 +3,9 @@
 @brief feature/dirauth: Directory authority implementation.
 
 This module handles running Tor as a directory authority.
+
+The directory protocol is specified in
+[dir-spec.txt](https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt).
+
+
 **/

src/feature/hs/feature_hs.dox

@@ -2,4 +2,9 @@
 @dir /feature/hs
 @brief feature/hs: v3 (current) onion service protocol
 
+This directory implements the v3 onion service protocol,
+as specified in
+[rend-spec-v3.txt](https://gitweb.torproject.org/torspec.git/tree/rend-spec-v3.txt).
+
+
 **/

src/feature/rend/feature_rend.dox

@@ -2,4 +2,8 @@
 @dir /feature/rend
 @brief feature/rend: version 2 (old) hidden services
 
+This directory implements the v2 onion service protocol,
+as specified in
+[rend-spec-v2.txt](https://gitweb.torproject.org/torspec.git/tree/rend-spec-v2.txt).
+
 **/

src/mainpage.dox

@@ -1,9 +1,7 @@
 /**
 @mainpage Tor source reference
 
-@section intro Getting to know Tor
-
-Welcome!
+@section intro Welcome to Tor
 
 This documentation describes the general structure of the Tor codebase, how
 it fits together, what functionality is available for extending Tor, and
@@ -22,7 +20,9 @@ This document is not an overview of the Tor protocol.  For that, see the
 design paper and the specifications at https://spec.torproject.org/ .
 
 For more information about Tor's coding standards and some helpful
-development tools, see doc/HACKING in the Tor repository.
+development tools, see
+[doc/HACKING](https://gitweb.torproject.org/tor.git/tree/doc/HACKING) in the
+Tor repository.
 
 @section highlevel The very high level