[plasmashell] Update docs for panelTakesFocus to make it generic

Summary:
We want surfaces with a notification role to be able to switch ability
to take focus or not on the fly. The semantics are identical as the
panel
taking focus, we may as well re-use what exists and remove the implicit
limitation than re-invent the wheel and complicate things.

Test Plan: Docs

Reviewers: #kwin, romangg

Reviewed By: #kwin, romangg

Subscribers: romangg, apol, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D26175