ACPI: add documentation for deprecated /proc/acpi/battery in ACPI_PROCFS

Add documentation in Kconfig help about the move of /proc/acpi/battery
to /sys/class/power_supply when selecting ACPI_PROCFS.  This will impact
a lot of users and should be documented.

Signed-off-by: Jerome Pinot <ngc891@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>