acpi_extract.py: document DEVICE directives

commit 2b568ebb20b08c7881d976b5bc9f59a425bae5e6
    acpi_extract: detect DeviceOp
added new directives to acpi_extract.py, but didn't
document them.
Add documentation at top of file.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
diff --git a/tools/acpi_extract.py b/tools/acpi_extract.py
index ab8ced6..8975b31 100755
--- a/tools/acpi_extract.py
+++ b/tools/acpi_extract.py
@@ -29,6 +29,9 @@
 # ACPI_EXTRACT_PROCESSOR_START - start of Processor() block
 # ACPI_EXTRACT_PROCESSOR_STRING - extract a NameString from Processor()
 # ACPI_EXTRACT_PROCESSOR_END - offset at last byte of Processor() + 1
+# ACPI_EXTRACT_DEVICE_START - start of Device() block
+# ACPI_EXTRACT_DEVICE_STRING - extract a NameString from Device()
+# ACPI_EXTRACT_DEVICE_END - offset at last byte of Device() + 1
 # ACPI_EXTRACT_PKG_START - start of Package block
 #
 # ACPI_EXTRACT_ALL_CODE - create an array storing the generated AML bytecode