Generate ejabberd.yml.5 man page from source code directly

Several documentation callbacks (doc/0 and mod_doc/0) are implemented
and `ejabberdctl man` command is added to generate a man page. Note
that the command requires a2x to be installed (which is a part of
asciidoc package).

src/mod_http_api.erl

@@ -30,12 +30,13 @@
 -behaviour(gen_mod).
 
 -export([start/2, stop/1, reload/3, process/2, depends/2,
-	 mod_options/1]).
+	 mod_options/1, mod_doc/0]).
 
 -include("xmpp.hrl").
 -include("logger.hrl").
 -include("ejabberd_http.hrl").
 -include("ejabberd_stacktrace.hrl").
+-include("translate.hrl").
 
 -define(DEFAULT_API_VERSION, 0).
 
@@ -520,3 +521,8 @@ hide_sensitive_args(NonListArgs) ->
 
 mod_options(_) ->
     [].
+
+mod_doc() ->
+    #{desc =>
+          ?T("This module provides a ReST API to call "
+             "ejabberd commands using JSON data.")}.