[Python] Clarify use of unit conversion functions

interfaces/cython/cantera/reaction.pyx

@@ -742,9 +742,9 @@ cdef class ExtensibleRate(ReactionRate):
     of the rate parameterization and its corresponding data class, and to make these
     rates constructible through factory functions and input files.
 
-    Classes derived from `ExtensibleRate` should implement the `set_parameters` and
-    `eval` methods, which will be called as delegates from the C++ :ct:`ReactionRate`
-    class.
+    Classes derived from `ExtensibleRate` should implement the `set_parameters`,
+    `get_parameters`, `eval`, and (optionally) `validate` methods, which will be called
+    as delegates from the C++ :ct:`ReactionRate` class.
 
     **Warning:** The delegatable methods defined here are an experimental part of the
     Cantera API and may change without notice.
@@ -779,13 +779,23 @@ cdef class ExtensibleRate(ReactionRate):
         for reactions created from YAML, ``params`` is the YAML reaction entry converted
         to an ``AnyMap``. ``rate_coeff_units`` specifies the units of the rate
         coefficient.
+
+        Input values contained in ``params`` may be in non-default unit systems,
+        specified in the user-provided input file. To convert them to Cantera's native
+        mks+kmol unit system, use the functions `AnyMap.convert`,
+        `AnyMap.convert_activation_energy`, and `AnyMap.convert_rate_coeff` as needed.
         """
         raise NotImplementedError(f"{self.__class__.__name__}.set_parameters")
 
     def get_parameters(self, params: AnyMap) -> None:
         """
         Responsible for serializing the state of the ExtensibleRate object, using the
         same format as a YAML reaction entry. This is the inverse of `set_parameters`.
+
+        Serialization methods may request output in unit systems other than Cantera's
+        native mks+kmol system. To enable conversions to the user-specified unit system,
+        dimensional values should be added to ``params`` using the methods
+        `AnyMap.set_quantity` and `AnyMap.set_activation_energy`.
         """
         raise NotImplementedError(f"{self.__class__.__name__}.get_parameters")
 

interfaces/cython/cantera/units.pyx

@@ -96,6 +96,9 @@ cdef class UnitSystem:
     native unit system, which is SI units except for the use of kmol as the base
     unit of quantity, that is, kilogram, meter, second, kelvin, ampere, and kmol.
 
+    Generally, this class is used indirectly, through methods interacting with `AnyMap`
+    objects such as `ExtensibleRate.set_parameters` and `ExtensibleRate.get_parameters`.
+
     The default unit system used by Cantera is SI+kmol::
 
         ct.UnitSystem({