ACPICA: Table Manager: Update comments in a function header

Update acpi_get_table_header to remove text stating that the caller
must unmap any returned memory.

ACPICA commit 4f3a235cf0044b2d91958b1f99b4ca824c63f948

Link: https://github.com/acpica/acpica/commit/4f3a235c


Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Erik Kaneda <erik.kaneda@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>