Add some more type notes

doc/types.rst

@@ -2,6 +2,11 @@
 Duktape typing
 ==============
 
+Typing overview
+===============
+
+TBD.
+
 Internal and external type summary
 ==================================
 
@@ -123,3 +128,31 @@ API and internals.  What's missing from the table:
 +----------------------------+---------------------+------------------------+-----------------------+-------------------------+-------------------------------------+-------------------+-------------------------------+-----------------------------------+
 | new Float64Array(1)        | DUK_TYPE_OBJECT     | duk_is_object()        | DUK_TAG_OBJECT        | duk_hbufobj             | DUK_HOBJECT_CLASS_FLOAT64ARRAY      | object            | [object Float64Array]         |                                   |
 +----------------------------+---------------------+------------------------+-----------------------+-------------------------+-------------------------------------+-------------------+-------------------------------+-----------------------------------+
+
+Options for representing a value
+================================
+
+There are four basic alternatives to representing a value:
+
+* **A tagged type with no heap allocation**.  This is the lowest footprint
+  alternative, and memory usage is 8 bytes (for a packed ``duk_tval``) or
+  (typically) 16 bytes (for a non-packed ``duk_tval``).  Example: undefined,
+  null, boolean, number, pointer.
+
+* **A heap allocated custom struct**.  A tagged value points to a heap
+  allocated C struct which is customized for a certain purpose.  Flags in
+  the object header allow a base C struct to be extended in certain cases.
+  Example: fixed buffer, dynamic buffer, external buffer, string.
+
+* **A heap allocated object**.  A tagged value points to a ``duk_hobject``.
+  Because a ``duk_hobject`` has a property table, type specific values can
+  be easily added to the property table, but properties have a relatively
+  high cost.  Example: plain Ecmascript object.
+
+* **A heap allocated extended object**.  A tagged value points to a struct
+  extending ``duk_hobject``.  Flags in the shared ``duk_hobject`` header
+  allow Duktape internals to detect the extended type and to access further
+  fields in an extended C struct.  The extended values may only be available
+  internally, but may also be accessible via property reads if the properties
+  are virtualized.  Example: Ecmascript function, Duktape/C function, thread,
+  buffer object.