docs: record why mail-overview counts by tag, not path

qtmaildir scopes an account with a path glob, so the obvious move was to copy
that query. It is wrong for this panel. notmuch deduplicates by message id, so
a message that arrived at two configured addresses is one message with two
paths, and a path glob counts it under both accounts: the five accounts here
sum to 102 against a global total of 101.

The icon shows one global total and the panel header repeats it, so rows that
sum to a different number would look like a bug. Counting by the account tag,
which is a property of the message and therefore singular, makes the rows sum
to the header. The cost is that a cross posted message appears under only the
account the post-new hook attributed it to.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WWL8JYHu7yhAdtx5pU9PMU

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