Added MACRO to docs for mpn_extras macros.
Added MACRO to docs for mpn_extras macros.

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