Split the doxygen doc for HypreParMatrix::LeftDiagMult() into "@brief" and full description.
Split the doxygen doc for HypreParMatrix::LeftDiagMult() into
"@brief" and full description.

mfem/mfem,mfem/mfem,mfem/mfem,mfem/mfem,mfem/mfem