Update bindable proerty docs after all changes

With the latest updates to bindable properties we need to revise the docs.
Some obvious fixes are:

• Make sure that the docs do not mention lazy evaluation.
• markDirty() is replaced with notify().
• Make QObjectComputedProperty docs public (as the class is in the public header).
• Update https://doc-snapshots.qt.io/qt6-dev/bindableproperties.html to mention QObjectComputedProperty as well.