fix(docs): link to included LTN documents (#426)

docs/ltn12.html

@@ -40,7 +40,7 @@
 <h2 id="ltn12">LTN12</h2>
 
 <p> The <tt>ltn12</tt> namespace implements the ideas described in
-<a href="http://lua-users.org/wiki/FiltersSourcesAndSinks">
+<a href="https://github.com/lunarmodules/luasocket/blob/master/ltn012.md">
 LTN012, Filters sources and sinks</a>. This manual simply describes the
 functions. Please refer to the LTN for a deeper explanation of the
 functionality provided by this module.

docs/socket.html

@@ -164,6 +164,9 @@ <h2 id="socket">The socket namespace</h2>
 <a href="#try"><tt>try</tt></a>
 function that allows for cleanup before the exception
 is  raised.
+This implements the ideas described in
+<a href="https://github.com/lunarmodules/luasocket/blob/master/ltn013.md">
+LTN012, Using finalized exceptions</a>.
 </p>
 
 <p class="parameters">
@@ -207,6 +210,9 @@ <h2 id="socket">The socket namespace</h2>
 function only catches exceptions thrown by the <a href="#try"><tt>try</tt></a>
 and <a href="#newtry"><tt>newtry</tt></a> functions. It does not catch normal
 Lua errors.
+This implements the ideas described in
+<a href="https://github.com/lunarmodules/luasocket/blob/master/ltn013.md">
+LTN012, Using finalized exceptions</a>.
 </p>
 
 <p class="parameters">
@@ -425,6 +431,9 @@ <h2 id="socket">The socket namespace</h2>
 Throws an exception in case <tt>ret<sub>1</sub></tt> is falsy, using
 <tt>ret<sub>2</sub></tt> as the error message. The exception is supposed to be caught
 by a <a href="#protect"><tt>protect</tt></a>ed function only.
+This implements the ideas described in
+<a href="https://github.com/lunarmodules/luasocket/blob/master/ltn013.md">
+LTN012, Using finalized exceptions</a>.
 </p>
 
 <p class="parameters">