Add a special description to summoned monsters.

This does introduce a balancing change since AFAIK it was previously impossible to distinguish between a summoned and a permanent monster. It's possible that this change leads to players always having to check a monster's description, in which case we might want to add a line to the monster's 'x' description, stating "It has been summoned."

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@8424 c06c8d41-db1a-0410-9941-cceddc491573

Created by  j-p-e-g  on January 12, 2009
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