Don't lie in the docs.

It seems we forgot to update the documentation when the default spacing
got changed to 5.

Change-Id: Id76ebd3d75ec5c0d5806aab65723d046c0f49864
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>
Support negative layout spacings (do not assert)

This tests
* default spacing
* explicit positive spacing
* negative spacings

Negative spacings that causes the layout to shrink down to 0 or
a negative spacing are not supported.
(But make sure they don't crash the whole engine)

Task-number: QTBUG-35741

Change-Id: I69521dffb03c35a63db37ab51a2e99152d88272b
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>