types: Added empty typemap attribute to CompositeType
types: Added docstring to GeometryDump.typemap
doc/types: Don't display undocumented members

We don't need to show that there is e.g. a result_processor() function
in the Raster type.
types: Adjusted GeometryDump typemap

The geometry_dump type consists of two fields:

- geom (holding a geometry object)
- path[] (a 1-d array holding the position of the geometry within
  the dumped object.)

see http://postgis.net/docs/geometry_dump.html