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
"unforeseeable": number of constrained degrees of freedom (unforeseeable from a mathematical point of view) minus multiplicities, which can only be determined by tracing what is going on deep inside dealii.

First subclause removed.