MAINTAINERS: add idr core-api doc file to XARRAY

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.

idr.rst provides some explanation to the ID allocation API defined in
lib/idr.c, which itself is part of the XARRAY section.

Add this core-api document to XARRAY.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Lukas Bulwahn <[e-mail]>
Cc: Matthew Wilcox (Oracle) <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

MAINTAINERS

···
 L:	[e-mail]
 L:	[e-mail]
 S:	Supported
+F:	Documentation/core-api/idr.rst
 F:	Documentation/core-api/xarray.rst
 F:	include/linux/idr.h
 F:	include/linux/xarray.h