doc: Re-organize rygel.conf man file

Add a "Plugin-specific settings" section title and move it so that
the plugin sections are directly below this section.
doc: Add documentation on diagnostics configuration item
core: Improve comment about queueing BasicManagement tasks
doc: Add BasicManagement README
core: return false in BasicManagement finish_iteration() source func
core: Use Rygel.StateMachine in BasicManagementTest