docs: reconcile backlog with usage notes, add items 39-45

The notes at the user's end are the upstream source and had drifted seven
entries ahead of this document. Each new item's cause is verified against
the code rather than restated from the note.

Two are worth separating from the rest:

- 41 is a defect. collectParts() files any part carrying a Content-Id into
  inlineParts and returns before the text/plain and text/html branches, so a
  message whose HTML body part has a content id parses with both body slots
  empty. That is one cause for both halves of the observation, a blank pane
  and "no HTML part".
- 44 cannot be planned yet. It asks to manage filters applied at sync time,
  and no such filters exist here: MailSync runs one command and the script is
  mbsync plus notmuch new. The item needs the user to say where their rules
  live before it has a shape.

Items 37 and 38 are deliberately left out of this commit; they document work
that is built but not yet hand-tested, and belong with that code.

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