add comment to explain using the LAMMPS_LIB_MPI define in example

doc/src/Library.rst

@@ -65,7 +65,7 @@ a problem when the communicator that would be passed is different from
 ``MPI_COMM_WORLD``.  Otherwise calling :cpp:func:`lammps_open_no_mpi`
 will work just as well.  To make :cpp:func:`lammps_open` available,
 you need to compile the code with ``-DLAMMPS_LIB_MPI`` or add the line
-``#define LAMMPS_LIB_MPI 1`` before ``#include "library.h"``.
+``#define LAMMPS_LIB_MPI`` before ``#include "library.h"``.
 
 Please note the ``mpi.h`` file must usually be the same (and thus the
 MPI library in use) for the LAMMPS code and library and the calling code.

doc/src/Library_execute.rst

@@ -32,8 +32,10 @@ Below is a short example using some of these functions.
 
 .. code-block:: C
 
+   /* define to make the otherwise hidden prototype for "lammps_open()" visible */
    #define LAMMPS_LIB_MPI
    #include "library.h"
+
    #include <mpi.h>
    #include <stdio.h>