acpi/acpigen: point out what acpigen_write_len_f does

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: Ibaf2f54f2f428f4438ef22b7f9d205db10e144db
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79001
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>

src/acpi/acpigen.c

@@ -29,6 +29,8 @@ void acpigen_write_len_f(void)
 {
 	ASSERT(ltop < (ACPIGEN_LENSTACK_SIZE - 1))
 	len_stack[ltop++] = gencurrent;
+	/* Reserve 3 bytes for PkgLength. The actual byte values will be written later in the
+	   acpigen_pop_len call. */
 	acpigen_emit_byte(0);
 	acpigen_emit_byte(0);
 	acpigen_emit_byte(0);