add a couple more versionadded directives

src/library.cpp

@@ -263,6 +263,8 @@ The MPI standard requires that any MPI application must call
 calls.  This function checks, whether MPI is already initialized and
 calls ``MPI_Init()`` in case it is not.
 
+.. versionadded:: 15Sep2020
+
 \endverbatim */
 
 void lammps_mpi_init()
@@ -293,6 +295,8 @@ before exiting the program to wait until all (parallel) tasks are
 completed and then MPI is cleanly shut down.  After this function no
 more MPI calls may be made.
 
+.. versionadded:: 15Sep2020
+
 \endverbatim */
 
 void lammps_mpi_finalize()