Spell every document the emitter refused for a carriage return, a null character, a code-span line start or an empty text node

CHANGELOG.md

@@ -17,6 +17,14 @@
   `content` or `marks` is spelled `{attrs=empty}`, `{content=empty}` or `{marks=empty}`, `-0` is
   spelled `-0`, and a `codeBlock` of several text nodes is a fence per node. A `codeBlock` holding
   other than plain text nodes rides the block carry, where it was refused.
+- **Breaking:** `unspellable-character` and `unspellable-line-start` leave `ConvertErrorCode`, and
+  `adfToMarkdown` no longer refuses `unspellable-whitespace`. A carriage return in text is spelled
+  `
` and a null character `!adf:text{text="\u0000"}`; a code block holding either, a text node
+  holding no text or holding `content`, a paragraph line opening with a code span whose backticks
+  read back as a fence, and an `emoji`, `mention` or `status` whose `text` holds a line ending or a
+  null character ride the carry. See `MIGRATION.md`.
+- Fix a link `title` holding a null character: it is written as `!adf:link[text]{attrs}`, where it
+  read back as U+FFFD.
 - **Breaking:** `unspellable-link` leaves `ConvertErrorCode`; a link whose `href` or `title` no
   CommonMark escape spells is written as `!adf:link[text]{attrs}`.
 - **Breaking:** some directive refusals carry `malformed-directive` where they carried