docs: record four items the usage notes had and the backlog did not

The notes are the upstream source and keep growing, so the backlog goes
stale on its own between sessions. This pass found three unrecorded
entries, each with its cause verified in the code rather than copied from
the note:

- 49, sync always runs every account regardless of what changed. The
  account set is not a parameter anywhere on the path, and the shipped
  script hardcodes the whole-store sweep. Needs a decision first about
  whether an account key is also an mbsync channel name.
- 50, Esc blanks the pane but leaves the row selected. Built as specified
  by item 32; the user now wants both, as two actions rather than one.
- 51, clicking a subject scrolls the list sideways. Ordinary view
  auto-scroll, cosmetic.

52 came from the work itself rather than from the notes: the suite's
verdict depends on how it is invoked. test_querycompleter grabs a popup
and Wayland refuses a grabbing popup for a window that has never received
input, so it fails under ctest and passes when run with the offscreen
platform by hand. It cost a wrong diagnosis on the day it was found, an
unrelated change blamed for a failure that reproduces on a clean
checkout, because the comparison was run under two different plugins.
Recorded with the transientParent fix preferred over forcing the
platform, since a test that runs under the real plugin is worth more than
one that opts out.

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