Improve screen mixins documentation

[alexander-shustanov]

https://doc.cuba-platform.com/manual-7.0/screen_mixins.html

It is very convenient to extend screen mixins from com.haulmont.cuba.gui.screen.FrameOwner, as in this case we can get screen extensions with

Extensions.get(this, DeclarativeLoaderParametersState.class)

In all provided examples the first argument of the Extensions.get/Extensions.register methods is received from one of the Screen events. But it will be unavailable from any other methods of the mixin.