DOC: clarify matrix vs array in mmio docstrings

mmio docstrings and an error message seemed to imply use of np.matrix,
which is not the case.
Merge pull request #3310 from matthew-brett/clarify-mmio-matrices

DOC: clarify matrix vs array in mmio docstrings
ENH: sparse: add maximum and minimum binops to sparsetools
ENH: sparse: implement spmatrix.maximum and spmatrix.minimum
GEN: regenerate sparsetools