config: document --[no-]value

These options were introduced in a series of commits from
fe3ccc7 (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>

Author: LemmingAvalanche

Documentation/git-config.adoc

@@ -200,6 +200,14 @@ See also <<FILES>>.
 	section in linkgit:gitrevisions[7] for a more complete list of
 	ways to spell blob names.
 
+`--value=<pattern>`::
+`--no-value`::
+	With `get`, `set`, and `unset`, match only against
+	_<pattern>_. The pattern is an extended regular expression unless
+	`--fixed-value` is given.
++
+Use `--no-value` to unset _<pattern>_.
+
 --fixed-value::
 	When used with the `value-pattern` argument, treat `value-pattern` as
 	an exact string instead of a regular expression. This will restrict