config: document --[no-]show-names

These options were introduced in 4e51389000 (builtin/config:
introduce "get" subcommand, 2024-05-06) but not documented here.
Use the description from the source code.

Document this option and the negated form according to the current
convention.[1]

`--show-names` is also the default when `--get-regexp` is given.  But
don’t mention it here since all the deprecated modes are quarantined in
the “Deprecated Modes” section.

[1]: 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>

Documentation/git-config.adoc

@@ -259,6 +259,12 @@ Valid `<type>`'s include:
 	Output only the names of config variables for `list` or
 	`get`.
 
+`--show-names`::
+`--no-show-names`::
+	With `get`, show config keys in addition to their values. The
+	default is `--no-show-names` unless `--url` is given and there
+	are no subsections in _<name>_.
+
 --show-origin::
 	Augment the output of all queried config options with the
 	origin type (file, standard input, blob, command line) and