docs(how-to): update-an-instance names the interrupted required migration (#195)

The guide said a required migration is simply carried out, which
contradicts upgrade-instance.md: the agent can stop after the first
publish, leaving a valid intermediate state that migrate status resumes
and doctor warns about. The guide now says so without retelling the
instruction's steps.

Files changed:
- docs/how-to/update-an-instance.md

Co-Authored-By: Claude Code <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SnAJ7Z3CpVD3PRbN73QtU2

docs/how-to/update-an-instance.md

@@ -80,10 +80,19 @@ In the order you'll meet them:
 - **The session restart.** The running session still holds the *old* `AGENTS.md`, contracts and
   skills, so the agent asks you to restart it before migrating content. The new session picks up
   where the old one stopped.
-- **An offered migration.** A required migration is simply carried out. An *offered* one changes a
-  file the instance owns, so taking it or declining it is your decision.
+- **A required migration.** The agent carries it out, and may come back to you with a question
+  while doing so: a migration document can reach a point that is yours to decide, such as a page
+  that contradicts another.
+- **An offered migration.** It changes a file the instance owns, so taking it or declining it is
+  your decision.
 - **The second publish,** of the migrated content, behind the same gate.
 
+**A required migration can also be interrupted.** If the agent cannot complete one now, it stops
+after the first publish and leaves it. That is a valid state: new machinery, content still in its
+old form. `doctor` keeps warning until the migration is done, and a later session resumes with
+`tools/wikitool migrate status`. The agent never records a migration it did not carry out, because
+the content's version would then describe a shape the pages are not in.
+
 If a step fails for a reason that isn't obvious, the agent stops and offers you a bug report rather
 than improvising ([Troubleshoot](troubleshoot.md)).