config: document --[no-]value

config: document --[no-]value

These options were introduced in a series of commits from
fe3ccc7aab (Merge branch 'ps/config-subcommands', 2024-05-15).[1]
But they were not documented here.

Document this option and the negated form according to the current
convention.[2]

[1]: `--value` is a replacement for the `value-pattern`
    positional argument
[2]: https://lore.kernel.org/git/xmqqcyct1mtq.fsf@gitster.g/

Acked-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>