documentation for qmldir "depends" declaration

[ChangeLog][QtQml][qmldir] added the ability to declare dependencies
on other modules in a module definition qmldir file

Task-number: QTBUG-41489
Change-Id: Icd526bc7617bc3e4f725c7d008d0522a201cf645
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>