add documentation on the effect of ConstraintMatrix::distribute_local_to...

Documentation update highlighting the difference between the two ConstraintMatrix::distribute_local_to_global() functions for matrices and their effect on the spectrum of a matrix
* "not changing the condition number" -- compared to what? I think you want to say something like
  "While the matrix that results when using this function has the same condition number as the matrix you'd get without eliminating constraints, ..."

* "unforeseeable": It's actually quite foreseeable -- the number of spurious eigenvalues equals the number of constrained degrees of freedom. I think it would help to also add a reference to the discussion of this issue in step-??