qapi/char.json: minor doc rewording for `hub` device

Refine documentation for the hub device, specify the maximum.

Signed-off-by: Roman Penyaev <r.peniaev@gmail.com>
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: qemu-devel@nongnu.org
Message-ID: <20250219090607.559887-1-r.peniaev@gmail.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

qapi/char.json

@@ -337,7 +337,7 @@
 #
 # Configuration info for hub chardevs.
 #
-# @chardevs: List of chardev IDs, which should be added to this hub
+# @chardevs: IDs to be added to this hub (maximum 4 devices).
 #
 # Since: 10.0
 ##