Qt Quick Layouts: Some small documentation refinements.

Document the unreliability of width and height.

Move the docs on how preferred size is determined to the overview
document instead of keeping it in the documentation for the Layout.*
attached property documentation, because its scope extends beyond this
class.

Task-number: QTBUG-34045 (inspired by)

Change-Id: Ie16f7d151b867aa45caf8e8e589f92ffd82e3220
Reviewed-by: Nico Vertriest <nico.vertriest@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>