Deprecate types 12 and 13 in spec

Neither type has ever been produced by any writer or appeared in any
known database. Type 12 (data cache container) is superseded by pointer-
based deduplication. Type 13 (end marker) is superseded by metadata-
based section boundary calculation. Mark both as deprecated and explain
their original intent.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: oschwald

MaxMind-DB-spec.md

@@ -385,27 +385,23 @@ size can be zero.
 This type uses the same algorithm as maps for determining the length of a
 field's payload.
 
-### data cache container - 12
+### data cache container - 12 (deprecated)
 
-This is a special data type that marks a container used to cache repeated
-data. For example, instead of repeating the string "United States" over and
-over in the database, we store it in the cache container and use pointers
-*into* this container instead.
+This type is deprecated. It has never been used in any known database and
+readers are not expected to support it.
 
-Nothing in the database will ever contain a pointer to this field
-itself. Instead, various fields will point into the container.
+It was originally intended to mark a container of repeated data that a database
+dumper tool could skip. In practice, data deduplication is handled by pointers,
+making this type unnecessary.
 
-The primary reason for making this a separate data type versus simply inlining
-the cached data is so that a database dumper tool can skip this cache when
-dumping the data section. The cache contents will end up being dumped as
-pointers into it are followed.
+### end marker - 13 (deprecated)
 
-### end marker - 13
+This type is deprecated. It has never been used in any known database and
+readers are not expected to support it.
 
-The end marker marks the end of the data section. It is not strictly
-necessary, but including this marker allows a data section deserializer to
-process a stream of input, rather than having to find the end of the section
-before beginning the deserialization.
+It was originally intended to mark the end of the data section for stream-based
+deserialization. In practice, readers determine section boundaries from the
+metadata, making this type unnecessary.
 
 This data type is not followed by a payload, and its size is always zero.