Merge pull request #6121 from bangerth/composition

Also document boundary composition plugins.

doc/sphinx/user/extending/plugin-types/temp-bc.md

@@ -1,16 +1,26 @@
 
-# Temperature boundary conditions
+# Temperature and compositional boundary conditions
 
-The boundary conditions are responsible for describing the temperature values
-at those parts of the boundary at which the temperature is fixed.
+Boundary conditions for the temperature (and compositional fields, if
+present) are responsible for describing the fields' values at those
+parts of the boundary at which the temperature or composition is
+fixed, as well as at inflow boundaries.
 
-To implement a new boundary conditions model, you need to overload the
+To implement a new boundary conditions model for the temperature, you need to overload the
 `aspect::BoundaryTemperature::Interface` class and use the
 `ASPECT_REGISTER_BOUNDARY_TEMPERATURE_MODEL` macro to register your new class.
 The implementation of the new class should be in namespace
 `aspect::BoundaryTemperature`.
 
-The principle function you need to overload needs to provide the fixed temperature at a
+Correspondingly, for a boundary conditions model for compositional fields, you need to overload the
+`aspect::BoundaryComposition::Interface` class and use the
+`ASPECT_REGISTER_BOUNDARY_COMPOSITION_MODEL` macro to register your new class.
+The implementation of the new class should be in namespace
+`aspect::BoundaryComposition`.
+
+In either case,
+the principle function of the respective `Interface` class that you need to overload
+provides the prescribed temperature or composition at a
 given point. The boundary indicator of the particular
 piece of boundary on which the point is located is also given as a hint in
 determining where this point may be located; this may, for example, be used to