some doc updates for result structure

I have updated the doc page ResultStruct with description of

• negative values of r.evals['df'], r.evals['dc'], r.evals['dh'] (it means they have been obtained via finite-difference approximation; in the case we take into account for r.evals['f'] all f calls - both from objFunc and from finite-difference derivatives approximation)
  
• r.iterValues.rt, r.iterValues.ri (type and index of max residual).