Add narative to forces

docs-source/api-c++/forces.rst.jinja2

 Forces
 ~~~~~~
 
+The Force objects added to a :cpp:obj:`System` define the behavior of the
+particles.  Force is an abstract class; subclasses implement specific behaviors.
+The Force class is actually slightly more general than its name suggests.  A
+Force can, indeed, apply forces to particles, but it can also directly modify
+particle positions and velocities in arbitrary ways.  Some thermostats and
+barostats, for example, can be implemented as Force classes.
+
 .. autodoxysummary::
     :toctree: generated/