Fixed note taking (NOTE_HP_CHANGE) for invisible monsters

(bug 1786776) and in general (wasn't working).

Also tweaked tutorial message on burning spellbooks (Bug 1799350).

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@2174 c06c8d41-db1a-0410-9941-cceddc491573

Created by  j-p-e-g  on September 21, 2007
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