Document the fact, that openFileBlocking can consume an OS thread indefinitely.
Document the fact, that openFileBlocking can consume an OS thread indefinitely.

Also state that a deadlock can happen with the non-threaded runtime.
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sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc