Update compound mapping docs

The documentation for CmpMap, SeriesMap and ParallelMap
was incorrect regarding the effects of the latter two
being a wrapper around the first.

include/astshim/CmpMap.h

@@ -52,6 +52,10 @@ way.
 ### Attributes
 
 @ref CmpMap has no attributes beyond those provided by @ref Mapping and @ref Object.
+
+@warning CmpMap will sometimes appears as a SeriesMap or ParallelMap, as appropriate, including:
+- getClassName() will return "SeriesMap" or "ParallelMap", as appropriate
+- A CmpMap persisted using a Channel or pickle will be unpersisted as a SeriesMap or ParallelMap
 */
 class CmpMap : public Mapping {
     friend class Object;

include/astshim/ParallelMap.h

@@ -41,7 +41,10 @@ component in forming further @ref ParallelMap "ParallelMaps".
 @ref Mapping "Mappings" of arbitrary complexity may be built from simple
 individual @ref Mapping "Mappings" in this way.
 
-@warning This wraps AST's AstCmpMap, so getClassName() returns "CmpMap".
+@warning ParallelMap is a convenience wrapper around CmpMap. Specialized code hides some
+of this, so getClassName() will return "ParallelMap" and an ParallelMap persisted using a Channel
+or pickle will be returned as a "ParallelMap" in Python. However, it will be visible in
+other ways, such as the output from show().
 
 ### Attributes
 

include/astshim/SeriesMap.h

@@ -41,7 +41,10 @@ Since a SeriesMap is itself a Mapping, it can be used as a
 component in forming further @ref SeriesMap "SeriesMaps". @ref Mapping "Mappings" of arbitrary
 complexity may be built from simple individual @ref Mapping "Mappings" in this way.
 
-@warning This wraps AST's AstCmpMap, so getClassName() returns "CmpMap".
+@warning SeriesMap is a convenience wrapper around CmpMap. Specialized code hides some
+of this, so getClassName() will return "SeriesMap" and an SeriesMap persisted using a Channel
+or pickle will be returned as a "SeriesMap" in Python. However, it will be visible in
+other ways, such as the output from show().
 
 ### Attributes