Expand the doc and add notes

doc/development/overview.rst

@@ -101,11 +101,12 @@ pipeline stages are executed.  Most functions receive a PointTableRef object,
 which refers to the active point table.  A PointTableRef can be stored
 or copied cheaply.
 
-A subclass of PointTable called StreamPointTable exists to allow a pipeline
-to run without loading all points in memory.  A StreamPointTable holds a
+A subclass of PointTable called FixedPointTable exists to allow a pipeline
+to run without loading all points in memory.  A FixedPointTable holds a
 fixed number of points.  Some filters can't operate in streaming mode and
 an attempt to run a pipeline with a stage that doesn't support streaming
-will raise an exception.
+will raise an exception. A custom implementation of this can be created
+by inheriting from a class called StreamPointTable.
 
 Point View
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

pdal/PointTable.hpp

@@ -274,6 +274,8 @@ class PDAL_DLL StreamPointTable : public SimplePointTable
     std::vector<bool> m_skips;
 };
 
+// A concrete implementation of StreamPointTable that uses a fixed-size
+// buffer for point data.
 class PDAL_DLL FixedPointTable : public StreamPointTable
 {
 public: