MAINTAINERS: add printk core-api doc file to PRINTK

The files in Documentation/core-api/ are by virtue of their top-level
directory part of the Documentation section in MAINTAINERS. Each file in
Documentation/core-api/ should however also have a further section in
MAINTAINERS it belongs to, which fits to the technical area of the
documented API in that file.

The printk.rst provides some explanation to the printk API defined in
include/linux/printk.h, which itself is part of the PRINTK section.

Add this core-api document to PRINTK.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@redhat.com>
Link: https://patch.msgid.link/20251105102832.155823-1-lukas.bulwahn@redhat.com
Signed-off-by: Petr Mladek <pmladek@suse.com>

Author: bulwahn

MAINTAINERS

@@ -20505,6 +20505,7 @@ R:	John Ogness <john.ogness@linutronix.de>
 R:	Sergey Senozhatsky <senozhatsky@chromium.org>
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux.git
+F:	Documentation/core-api/printk-basics.rst
 F:	include/linux/printk.h
 F:	kernel/printk/