docs: record how to find the messages item 41 affected

Confirmed on real mail: a message whose text/html part carries a
Content-Id now renders. Item 41's status drops the caveat about the
reported message not being relocated.

Finding an affected message is harder than it looks, which is why the
search is worth recording rather than rederiving. A Content-Id on an
inline image is the common case and always worked, so a plain grep for
the header returns mostly noise. The narrow case is a Content-Id in the
same header block as a text/html Content-Type, which needs a small awk
pass over the message files. That found 96 in one inbox, all bulk
senders.

Also records that file: is not a notmuch search term, so a matched path
cannot be turned into a query directly; the Message-ID header is the
way across to an id: query.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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