Doc: Remove QX11EmbedContainer/QX11EmbedWidget snippets

- These classes no longer exist in Qt 5
- These snipets are not actually used in current docs

Change-Id: I6e83052c66c438be468f75f049041ab9fc0b5d20
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Explain what ICU is used for on Windows.

Task-number: QTBUG-28766
Change-Id: I5e964713c8f421a5a977cb760c0178f90404b8e1
Reviewed-by: John Layt <jlayt@kde.org>