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
I misread that you were talking about square systems here (I was still in the rectangular mode from the paragraph above). So the issue with diag_element() is moot. Thanks for adding the issue with Au=f.