[DOC] compleate function documentation and change API for interface

doc/mainpage.md

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+ESIGNAL library                                {#mainpage}
+===============
+
+What is ESIGNAL, and how can I use it?
+----------------------------------
+
+ESIGNAL, or Ewol signal engine is a simple messaging layer, managing multiple connection and manage disconnection
+
+ESIGNAL is designed for
+  - Expose signal
+  - Emit synchronous signal (not send through an event loop but directly)
+  - Manage versatil connection/disconnection
+
+
+What languages are supported?
+-----------------------------
+
+ESIGNAL is written in C++.
+
+
+Are there any licensing restrictions?
+-------------------------------------
+
+ESIGNAL is **FREE software** and _all sub-library are FREE and staticly linkable !!!_
+
+
+License (APACHE-2.0)
+--------------------
+
+Copyright esignal Edouard DUPIN
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+<http://www.apache.org/licenses/LICENSE-2.0>
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+