Add a paragraph about allocation domains to the C-API docs

Doc/c-api/memory.rst

@@ -92,6 +92,38 @@ for the I/O buffer escapes completely the Python memory manager.
    statistics of the :ref:`pymalloc memory allocator <pymalloc>` every time a
    new pymalloc object arena is created, and on shutdown.
 
+Allocator Domains
+=================
+
+All allocating functions belong to one of three different "domains" (see also
+:c:type`PyMemAllocatorDomain`). These domains represent different allocation
+strategies and are optimized for different purposes. The specific details on
+how every domain allocates memory or what internal functions each domain calls
+is considered an implementation detail, but for debugging purposes a simplified
+table can be found at :ref:`here <default-memory-allocators>`. There is no hard
+requirement to use the memory returned by the allocation functions belonging to
+a given domain for only the purposes hinted by that domain (although this is the
+recommended practice). For example, one could use the memory returned by
+:c:func:`PyMem_RawMalloc` for allocating Python objects or the memory returned
+by :c:func:`PyObject_Malloc` for allocating memory for buffers.
+
+The three allocation domains are:
+
+* Raw domain: intended for allocating memory for general-purpose memory
+  buffers where the allocation *must* go to the system allocator or where the
+  allocator can operate without the :term:`GIL`. The memory is requested directly
+  to the system.
+
+* "Mem" domain: intended for allocating memory for Python buffers and
+  general-purpose memory buffers where the allocation must be performed with
+  the :term:`GIL` held. The memory is taken from the Python private heap.
+
+* Object domain: intended for allocating memory belonging to Python objects. The
+  memory is taken from the Python private heap.
+
+When freeing memory previously allocated by the allocating functions belonging to a
+given domain,the matching specific deallocating functions must be used. For example,
+:c:func:`PyMem_Free` must be used to free memory allocated using :c:func:`PyMem_Malloc`.
 
 Raw Memory Interface
 ====================
@@ -601,4 +633,3 @@ heap, objects in Python are allocated and released with :c:func:`PyObject_New`,
 
 These will be explained in the next chapter on defining and implementing new
 object types in C.
-