Several documentation enhancements and build warnings fixes

- Made the documentation even more friendly towards newbies.
- Eased the usage of methods like get history which now set
  a default empty message for message actions and vice versa.
- Fixed some docstring documentations too.
- Updated the old normal docs/ to link back and forth RTD.
- Fixed the version of the documentation, now auto-loaded.

readthedocs/telethon.rst

@@ -42,6 +42,13 @@ telethon\.utils module
     :undoc-members:
     :show-inheritance:
 
+telethon\.session module
+------------------------
+
+.. automodule:: telethon.session
+    :members:
+    :undoc-members:
+    :show-inheritance:
 
 telethon\.cryto package
 ------------------------
@@ -58,21 +65,21 @@ telethon\.errors package
     telethon.errors
 
 telethon\.extensions package
-------------------------
+----------------------------
 
 .. toctree::
 
     telethon.extensions
 
 telethon\.network package
-------------------------
+-------------------------
 
 .. toctree::
 
     telethon.network
 
 telethon\.tl package
-------------------------
+--------------------
 
 .. toctree::