Update documentation

Author: connoramoreno

include/openmc/boundary_condition.h

@@ -149,7 +149,7 @@ class RotationalPeriodicBC : public PeriodicBC {
 };
 
 //==============================================================================
-//! A BC that transforms particle trajectory by a matrix.
+//! A BC that transforms particle direction and position by matrices.
 //==============================================================================
 
 class TransformationBC : public BoundaryCondition {

openmc/surface.py

@@ -127,7 +127,9 @@ class Surface(IDManagerMixin, ABC):
         surface. Defaults to transmissive boundary condition where particles
         freely pass through the surface. Note that periodic boundary conditions
         can only be applied to x-, y-, and z-planes, and only axis-aligned
-        periodicity is supported.
+        periodicity is supported. In addition, to enable greatest flexibility
+        for transformation boundary conditions, it is up to the user to ensure
+        valid transformations for transport.
     albedo : float, optional
         Albedo of the surfaces as a ratio of particle weight after interaction
         with the surface to the initial weight. Values must be positive. Only