Root finding page manual suggestion

[Brian Gough]

Francesco Potorti` writes:
 > I've used this  criterion in some small numerical  program I've written,
 > and it  works ok.   The above  means that Newton's  method wins  only if
 > computing f'(x) is  more than twice faster than  computing f(x), a quite
 > rare occurrence in practice.  I suggest this fact to be mentioned in the
 > manual.

Thanks for the information, I wasn't aware of that.  I'll add a note
in the manual.  

One factor in favor of Newton's method might be better numerical
stability on roots of higher multiplicity -- the subtraction of
f(a)-f(b) in the secant method could introduce a cancellation error
there.

regards
Brian Gough