bip138: clarify that 0x00 ends payload parsing

Specify that parsing stops immediately. Add a vector with an item followed
by a terminator and ignored bytes.

Clarify that TYPE 0x00 also doesn't require TYPE_LENGTH.

bip-0138.md

@@ -408,7 +408,7 @@ remaining bytes are `PADDING`. A `PAYLOAD` MUST contain at least one item (see
 
 `TYPE_LENGTH`: variable-length integer representing the length of `TYPE_PARAMS` in bytes.
 
-For all `TYPE` values except `0x01`, `TYPE_LENGTH` MUST be present.[^length-required]
+For all `TYPE` values except `0x00` and `0x01`, `TYPE_LENGTH` MUST be present.[^length-required]
 
 [^length-required]: **Why is `TYPE_LENGTH` mandatory?**
     `TYPE_LENGTH` is what lets a parser skip a content type it does not implement and still
@@ -417,20 +417,19 @@ For all `TYPE` values except `0x01`, `TYPE_LENGTH` MUST be present.[^length-requ
     implementation that only handles descriptors from recovering that descriptor. A
     type whose `TYPE_PARAMS` is always empty still carries a zero `TYPE_LENGTH` for this reason.
     `0x01` is exempt because every implementation handles it, so it is never skipped
-    blindly.
+    blindly. `0x00` is a terminator rather than a content item.
 
 `TYPE_PARAMS`: variable-length field whose encoding depends on `TYPE`.
 
 For `TYPE` values defined above:
-- 0x00: parsers MUST stop reading content items and treat the remaining bytes as
+- 0x00: parsers MUST stop immediately and treat the remaining bytes as padding.
-  padding.
 - 0x01: `TYPE_LENGTH` MUST be omitted and `TYPE_PARAMS` is a 2-byte big-endian unsigned integer
   representing the BIP number that defines it.
 - 0x02: `TYPE_PARAMS` MUST be `TYPE_LENGTH` bytes of opaque, vendor-specific data.
 - 0x03: `TYPE_PARAMS` MUST be empty, and the following `CONTENT` is the string itself,
   which MUST be valid UTF-8.
 
-For all `TYPE` values except `0x01`, parsers MUST reject `CONTENT_TYPE` if `TYPE_LENGTH` exceeds
+For all `TYPE` values except `0x00` and `0x01`, parsers MUST reject `CONTENT_TYPE` if `TYPE_LENGTH` exceeds
 the remaining payload bytes.
 
 For an unknown `TYPE` less than `0x80`, parsers MUST consume its `TYPE_LENGTH` bytes of
@@ -595,6 +594,10 @@ The version field enables possible future enhancements:
 vectors for key serialisations.  
 [content_type.json](./bip-0138/test_vectors/content_type.json) contains test
 vectors for content types serialisations.  
+[payload.json](./bip-0138/test_vectors/payload.json) contains test vectors for payload parsing.
+`payload` and each item's `content` are hexadecimal bytes; `type` is the numeric
+type byte, and `items` lists the content items before the terminator.
+
 [derivation_path.json](./bip-0138/test_vectors/derivation_path.json) contains
 test vectors for derivation paths serialisations.  
 [individual_secrets.json](./bip-0138/test_vectors/individual_secrets.json)

bip-0138/test_vectors/payload.json

@@ -0,0 +1,13 @@
+[
+  {
+    "description": "Bytes after terminator are ignored",
+    "valid": true,
+    "payload": "030001610080ff",
+    "items": [
+      {
+        "type": 3,
+        "content": "61"
+      }
+    ]
+  }
+]