docs(const): state what _missing_ actually does instead of claiming it mints

docs/source/pcapkit/const/index.rst

@@ -11,24 +11,42 @@ automatically generated from the :mod:`pcapkit.vendor` module.
 Unrecognised Values
 -------------------
 
-Every enumeration below departs from :mod:`enum` in one deliberate way, and it is
-the behaviour to know about before using any of them: looking one up by a value
-the registry does not define does **not** raise. Each class overrides
-``_missing_`` so that a value inside the registry's valid range is minted into a
-new member on the fly -- via ``aenum.extend_enum``, named for the unassigned or
-reserved band it falls in -- and returned. Only a value outside that range, or of
-the wrong type, raises :exc:`ValueError`.
+Nearly every registry below departs from :mod:`enum` in one deliberate way, and it
+is the behaviour to know about before using any of them: looking one up by a value
+it does not define does **not** raise. Its ``_missing_`` override returns a
+member-like object for a value inside the registry's valid range, named for the
+unassigned or reserved band it falls in. Only a value outside that range, or of the
+wrong type, raises :exc:`ValueError`.
+
+In almost every registry that object is **not** installed: the registry does not
+grow, and a second lookup of the same value returns an equal but distinct object.
+A handful behave otherwise, and :ref:`mint-criterion` gives the full split:
+
+* :class:`~pcapkit.const.mh.cga_type.CGAType` installs a permanent member, so
+  ``__members__`` and iteration grow.
+* :class:`~pcapkit.const.tcp.flags.Flags` and the four Mobility Header flag
+  registries, such as
+  :class:`~pcapkit.const.mh.binding_ack_flag.BindingACKFlag`, hand back to
+  ``aenum``'s flag base, which caches the composed object in the value lookup
+  table only: a second lookup returns the same object, while ``__members__``
+  and iteration are unchanged.
+* :class:`~pcapkit.const.hip.transport.Transport`,
+  :class:`~pcapkit.const.ipv6.extension_header.ExtensionHeader`,
+  :class:`~pcapkit.const.pcapng.tls_key_label.TLSKeyLabel` and the memberless
+  :class:`~pcapkit.const.reg.apptype.apptype.AppType` base raise
+  :exc:`ValueError` for any unknown value.
 
 That is what makes the enumerations usable against live capture data, where a
 protocol number assigned after this release was generated is a routine
 occurrence rather than an error. It also means an enumeration member is not a
-closed set: the identity of a minted member is stable for the life of the
-process, but it does not exist until something asks for it.
-
-The mechanism is uniform because it is generated -- see
-:mod:`pcapkit.vendor.default`, whose template emits the ``_missing_`` override
-for every registry -- while the valid range and the names of the unassigned bands
-are per-registry, taken from that registry's own IANA data. The individual
+closed set: an unassigned value resolves to a member that is, apart from
+``CGAType``, absent from iteration and ``__members__``, and whether a repeat
+lookup returns the same object varies by registry, so compare such members by
+value, not identity.
+
+The common form of the override is generated -- see :mod:`pcapkit.vendor.default`,
+whose template emits it -- while the valid range and the names of the unassigned
+bands are per-registry, taken from that registry's own IANA data. The individual
 overrides are therefore not documented per class.
 
 .. seealso::