added and fixed comments in rref_mul
added and fixed comments in rref_mul

dsroche/flint2,fredrik-johansson/flint2,dsroche/flint2,wbhart/flint2,dsroche/flint2,dsroche/flint2,jpflori/flint2,fredrik-johansson/flint2,jpflori/flint2,jpflori/flint2,jpflori/flint2,wbhart/flint2,wbhart/flint2,fredrik-johansson/flint2