harmonize all plugins docs

original issue #1012
 
Add a configuration section that lists all options for each plugin.
List options in alphabetic order.
Mention the default value for each option.
Use same sentences to describe concepts common to different plugins eg 'auto' option, install description
Here's an example where I liked the old title better, even though it's inconsistent—I think it's OK to have a more descriptive heading than "Usage" *when appropriate*. Of course, when there's nothing specific to be said, we should stick with a standard.