added docu for 2-1 division with precomputed inverse
added docu for 2-1 division with precomputed inverse

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