Core: Adapt documentation to recent refactoring.

ProgressManager and ActionManager are no longer retrieved via ICore,
and most accesses do not go via the instance anymore.

Change-Id: Ib0c3033270051aaadfb25e0b2b0eaf7bdec63a1d
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>