doc: Better explain which complex result is chosen by .^ * expr.txi: Explain which complex root is chosen and how to choose a real one if preferred. * data.cc (power): Ditto. * help.cc (.^, .**): Ditto. (^, **): Suggest using real-valued functions.

#include "mex.h"

void
mexFunction (int nlhs, mxArray *plhs[], int nrhs, 
             const mxArray *prhs[])
{
  mxArray *v = mxCreateDoubleMatrix (1, 1, mxREAL);
  double *data = mxGetPr (v);
  *data = 1.23456789;
  plhs[0] = v;
}