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.

I scanned through the documentation of steps 28 and 36, but I could not find a reference to this issue. Did I miss it?

First subclause removed.