PATCH: Add more documentation for prerequisites

[Mark Mitchell]

Prerequisites seem to confuse a lot of people; here's a documentation 
update that tries to clarify their purpose and the fact that you should 
not use prerequisite tests to try to make one test depend on 
side-effects from another test.

Committed.

-- 
Mark Mitchell
CodeSourcery, LLC
mark at codesourcery.com
(916) 791-8304
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