Mention that char_set = unicode works only if Crawl was built with Unicode

support.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@1553 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dshaligram  on June 7, 2007
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