Document BSON interoperability and subtype-less binary round trips (#5330)

- warn about BSON marker 0x11 interoperability in both directions
- explain subtype-less binary normalization to subtype 0x00
- add a round-trip test for binary values without a subtype

Signed-off-by: YingqiDuan <141370165+YingqiDuan@users.noreply.github.com>

docs/mkdocs/docs/features/binary_formats/bson.md

@@ -35,6 +35,19 @@ The library uses the following mapping from JSON values types to BSON types:
     The mapping is **incomplete**, since only JSON-objects (and things contained therein) can be serialized to BSON.
     Also, keys may not contain U+0000, since they are serialized a zero-terminated c-strings.
 
+!!! warning "BSON type 0x11 interoperability"
+
+    The BSON specification defines type `0x11` as a Timestamp. This library uses marker `0x11` when serializing
+    `number_unsigned` values in the range `9223372036854775808..18446744073709551615`. Other BSON implementations may
+    therefore interpret these values as Timestamps instead of unsigned integers.
+
+!!! info "Binary values without a subtype"
+
+    BSON requires every binary value to have a subtype. If a binary value has no subtype, this library serializes it
+    with the generic subtype `0x00`. After deserialization, `has_subtype()` returns `true` and `subtype()` returns `0`.
+    As a result, serializing and deserializing a JSON object containing such a value produces a different JSON object,
+    even though the binary data is unchanged.
+
 ??? example
 
     ```cpp
@@ -82,8 +95,8 @@ The library maps BSON record types to JSON value types as follows:
 
 !!! note "Handling of BSON type 0x11"
 
-    BSON type 0x11 is used to represent uint64 numbers. This library treats these values purely as uint64 numbers 
-    and does not parse them into date-related formats.
+    This library deserializes BSON type `0x11` (Timestamp) as a `number_unsigned` value. The 64-bit value is preserved,
+    but the Timestamp type information is not.
 
 ??? example