clarify explanation of body style molecule in rigid fixes

doc/src/fix_rigid.txt

@@ -212,8 +212,9 @@ pour"_fix_pour.html.
 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