Merge branch 'task-doc' into 'master'

Add doc about parallel Tasks. Fix some warning in plugins doc

See merge request simgrid/simgrid!173

diff --cc docs/source/Plugins.rst
index 683bb4f,5212c9c..659e67c
--- 1/docs/source/Plugins.rst
--- 2/docs/source/Plugins.rst
+++ b/docs/source/Plugins.rst
@@@ -224,15 -224,15 +224,15 @@@ Batter
  
  .. doxygengroup:: plugin_battery
  
 -.. _plugin_photovoltaic:
 +.. _plugin_solar_panel:
  
 -Photovoltaic
 +Solar Panel
  ===========
  
 -.. doxygengroup:: plugin_photovoltaic
 +.. doxygengroup:: plugin_solar_panel
  
  Chiller
- ===========
+ =======
  
  .. doxygengroup:: plugin_chiller