Haiku Book should warn about area_delete changes

[Barrett]

A comment in the current virtual memory code states that differently from BeOS, in Haiku it's not possible to delete an area_id created by another team, this should be cleared out in the Haiku Book.

[waddlesplash]

We aren't allowed to modify the Be Book, but we can (and should) add a note about this to the Haiku Book.

[Barrett]

I've seen in the bebook lines stroked for deprecated API, i've think that we can stroke in the bebook delete_area section the phrases "Currently, anybody can delete any area—the act isn't denied if, for example, the area_id argument was created by another application." and "Until then, try to", but maybe i'm wrong and those lines were already stroked.