MAINTAINERS: add ATA sysfs file documentation to libata entry

Add the (still missing!) ATA sysfs file documentation to the libata
subsystem entry in the MAINTAINERS file.

Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>

Author: Sergey Shtylyov

MAINTAINERS

@@ -11257,6 +11257,7 @@ M:	Damien Le Moal <damien.lemoal@opensource.wdc.com>
 L:	linux-ide@vger.kernel.org
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/dlemoal/libata.git
+F:	Documentation/ABI/testing/sysfs-ata
 F:	Documentation/devicetree/bindings/ata/
 F:	drivers/ata/
 F:	include/linux/ata.h