Document how -O3 is handled by GHC
Document how -O3 is handled by GHC

    -O2 is the highest value of optimization.
    -O3 will be reverted to -O2.

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc