clarify explanation of body style molecule in rigid fixes

For bodystyle {single} the entire fix group of atoms is treated as one

rigid body.  This option is only allowed for the {rigid} styles.

For bodystyle {molecule}, each set of atoms in the fix group with a

different molecule ID is treated as a rigid body.  This option is

For bodystyle {molecule}, atoms are grouped into rigid bodies by their

respective molecule IDs: each set of atoms in the fix group with the

same molecule ID is treated as a different rigid body.  This option is

allowed for both the {rigid} and {rigid/small} styles.  Note that

atoms with a molecule ID = 0 will be treated as a single rigid body.

For a system with atomic solvent (typically this is atoms with