doc: add incompatibilities news item for create_(boundary_;)mass_matrix

diff --git a/doc/news/changes/incompatibilities/20180525Maier-2 b/doc/news/changes/incompatibilities/20180525Maier-2
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+Changed: MatrixCreator::create_mass_matrix and
+MatrixCreator::create_boundary_mass_matrix that had mixed number type (real
+valued for the matrix and complex valued for vectors) have been changed to
+support complex number types uniformly. This implies that now all
+underlying number types of matrix, vectors and AffineConstraint objects
+have to match; mixed variants are no longer supported.
+<br>
+(Matthias Maier, 2018/05/25)