Harmonize plugins docs styles

related to issue #1012 

NRTM (not ready to merge), just a glimpse of the changes I intended with the 3 first plugins doc files.
Would appreciate a feedback, to see if the effort goes in the right direction.

Basically, changes are :  
- having two distincts *Usage* and *Configuration* sections. Sections are optional but if they exist, they must have that exact names.
- always use same sentence to explain how to add a plugin
- configuration should list all the options and eventually provide an example