Fix descriptions of zombified type-creating abilities to mention that

zombified types can't leave their original level via stairs. They can still leave it via banishment or shaft traps.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@8035 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on December 30, 2008
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