doc tweak on added lib functions

as a library, run LAMMPS on a subset of processors, grab data from

LAMMPS, change it, and put it back into LAMMPS.

library.cpp contains the following functions for creating and

destroying an instance of LAMMPS and sending it commands to execute.

See the documentation in the src/library.cpp file for details.

The file src/library.cpp contains the following functions for creating

and destroying an instance of LAMMPS and sending it commands to

execute.  See the documentation in the src/library.cpp file for

details.

NOTE: You can write code for additional functions as needed to define

how your code talks to LAMMPS and add them to src/library.cpp and

comments in src/library.cpp file for which other functions need this

clean-up.

library.cpp also contains these functions for extracting information

from LAMMPS and setting value within LAMMPS.  Again, see the

documentation in the src/library.cpp file for details, including

The file src/library.cpp also contains these functions for extracting

information from LAMMPS and setting value within LAMMPS.  Again, see

the documentation in the src/library.cpp file for details, including

which quantities can be queried by name:

int lammps_extract_setting(void *, char *)