Add details about newest features in the documentation

[Guts]

Feature description

For now, the documentation does not cover the newest features introduced in version 2.2.

A quick list about what it's missing:

• describe how and when menus are generated in a classic QGIS workflow (threaded, in // of plugin init? after ui has finished?
• cache mechanism. This one requires to write about some concepts et give examples.

Additional context

This feature request is formulated from Agences de l'Eau and ported by Oslandia.