Tell migrators an empty stored document gets content: [], and what an adf: fence refuses

MIGRATION.md

@@ -6,7 +6,8 @@ Directives moved under the `!adf:` prefix. `0.2.0` reads `0.1.0`'s spelling with
 turning each directive into text and each carried node into an `adf` code block. Before `0.2.0`
 reads any `0.1.0` markdown, convert what is stored or in flight (an open editor, a queue) with the
 recipe below, and rewrite markdown your code writes or matches (templates, prompts, patterns) by
-the tables below. Stored ADF needs no change.
+the tables below. Stored ADF needs one change: a document `0.1.0` read from empty markdown holds
+no `content` key, and gets `content: []`, the empty document it meant.
 
 ### Convert markdown
 
@@ -30,8 +31,9 @@ function migrateMarkdown(stored: string) {
 - Convert each document once: a second pass can return ok while turning the directives into text.
   Stop `0.1.0` writing first, and record which documents are converted.
 - A refusal carrying `position` is `0.1.0`'s parse, which refused that markdown before too. One
-  without is `0.2.0`'s emit: store the document `markdownToAdf010` read as ADF rather than keeping
-  the unconverted markdown.
+  without is `0.2.0`'s emit: store the document `markdownToAdf010` read as ADF, with
+  `content: parsed.value.content ?? []` as the recipe gives it, rather than keeping the unconverted
+  markdown.
 
 ### Spellings
 
@@ -56,7 +58,7 @@ Markdown the spelling table leaves alone, which `0.2.0` reads as a different doc
 | --- | --- | --- |
 | a link whose text already holds one (`[a<https://example.com/>b](/v)`) | marks every node the inner link does not, splitting the outer link around it | leaves the outer brackets literal text; write the pieces as separate links to keep them |
 | markdown holding no block (`markdownToAdf("")`) | `{ type: 'doc', version: 1 }` | `{ content: [], type: 'doc', version: 1 }`; `!adf:doc {content=none}` reads as the former |
-| a code fence whose info string opens `adf:` (```` ```adf:x ````) | a `codeBlock` with that language | the block carry; write `!adf:codeBlock {language="adf:x"}` around a bare fence to keep the code block |
+| a code fence whose info string opens `adf:` (```` ```adf:x ````) | a `codeBlock` with that language | the block carry, refusing a body that is not one node's canonical JSON; write `!adf:codeBlock {language="adf:x"}` around a bare fence to keep the code block |
 
 ### Error codes