Make shadow dragons' corpses mutagenic as discussed some time ago, and

mark skeletal dragons' genus as "dragon", so they are affected by dragon slaying. Add several more descriptions for various abilities. Please feel free to correct or add anything.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@6261 c06c8d41-db1a-0410-9941-cceddc491573

Created by  j-p-e-g  on June 30, 2008
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