documentation: Merge awful.client and client pages

ldoc doesn't allow to specify fields from class "A" into class "B",
so the only solution is to merge the 2.

Also, one of the most common complain on IRC since Awesome 3.0 is
that the client API doc is  confusing since it is in 2 different files.

Also restore the `awful.client` doc link, point to `client`

This will avoid broken links.

docs/aliases/awful_client.lua

@@ -0,0 +1,6 @@
+---------------------------------------------------------------------------
+--- This module is deprecated, use `client`
+--  ===============================
+--
+-- @module awful.client
+---------------------------------------------------------------------------

docs/config.ld

@@ -63,6 +63,8 @@ file = {
     '../objects/',
     -- LUA libraries
     '../lib/',
+    -- Old APIs the user should not longer use directly
+    '../../docs/aliases/awful_client.lua',
     exclude = {
         -- exclude these modules, as they do not contain any written
         -- documentation

lib/awful/client.lua

@@ -4,7 +4,7 @@
 -- @author Julien Danjou <julien@danjou.info>
 -- @copyright 2008 Julien Danjou
 -- @release @AWESOME_VERSION@
--- @module awful.client
+-- @module client
 ---------------------------------------------------------------------------
 
 -- Grab environment we need