docs: make the mimetype completion list user-extensible

Mimetypes are the one completion list with no enumerator and an open-ended
set, so [completion] extra_mimetypes appends to the built-ins. Appending
rather than replacing means a typo or a short list cannot leave completion
worse off than the defaults.

Entries are comma-separated with an optional description after '|'. The two
separators differ because QSettings splits comma lists itself, so a
description containing a comma would otherwise be torn into two entries.

The other lists stay fixed: prefixes come from notmuch, paths from the
configured accounts, dates are closed once symbolic and relative forms are
covered, and tags are read from the database. A user-editable copy of any of
them would only drift from its source. For prefixes specifically, the upgrade
path is deriving the list from the installed notmuch, not making it editable.

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