Doc: Use critical sections for custom types

You should generally not use critical sections with built-in types
because they already use critical sections internally.

Doc/c-api/init.rst

@@ -2226,6 +2226,12 @@ is resumed, and its locks reacquired.  This means the critical section API
 provides weaker guarantees than traditional locks -- they are useful because
 their behavior is similar to the :term:`GIL`.
 
+Critical sections are appropriate for protecting custom types defined by your
+C-API extensions. They should generally not be used with built-in types like
+:c:struct:`PyDictObject` or :c:struct:`PyListObject` because their public APIs
+already use critical sections internally, with the notable exception of
+:c:func:`PyDict_Next`.
+
 The functions and structs used by the macros are exposed for cases
 where C macros are not available. They should only be used as in the
 given macro expansions. Note that the sizes and contents of the structures may