Extend description of BackgroundTask methods

[yuriy-khrebtov]

To describe execution contexts of BackgroundTask methods:

• run - executed in another thread that has no direct access to UI components, has no access to session data, has access to security context;
• other methods (progress, canceled, done etc.) - executed in a thread that has direct access to UI components, has access to session data, has access to security context

For the moment "executed in UI thread" statement leads to misunderstanding that these methods are invoked only by request from UI.