Tweak quiver command some more (allow clearing quiver).

Update documentation.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@6113 c06c8d41-db1a-0410-9941-cceddc491573

Created by  j-p-e-g  on June 24, 2008
4RJJ3ZFKNROSHTDWAM4ZCDQGM3R6SWGFRRAMML6GXPZ3466GLL5AC
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