added item in Efficiency section

git-svn-id: svn://scm.gforge.inria.fr/svn/mpfr/trunk@4895 280ebfd0-de03-0410-8827-d642c229c3f4

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   possible through stripping low zero bits or limbs could check for
   that (this would be less efficient but easier).
 
+- try the idea of the paper "Reduced Cancellation in the Evaluation of Entire
+  Functions and Applications to the Error Function" by W. Gawronski, J. Mueller
+  and M. Reinhard, to be published in SIAM Journal on Numerical Analysis: to
+  avoid cancellation in say erfc(x) for x large, they compute the Taylor
+  expansion of erfc(x)*exp(x^2/2) instead (which has less cancellation),
+  and then divide by exp(x^2/2) (which is simpler to compute).
 
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 6. Miscellaneous