2nd set of updates

doc/src/fix_modify.txt

doc/src/fix_poems.txt

doc/src/fix_rigid.txt

src/RIGID/fix_rigid.cpp

src/RIGID/fix_rigid.h

src/RIGID/fix_rigid_nh.cpp

src/RIGID/fix_rigid_nh_small.cpp

src/RIGID/fix_rigid_small.cpp

src/RIGID/fix_rigid_small.h

src/USER-OMP/fix_rigid_omp.h

src/USER-OMP/fix_rigid_small_omp.cpp