inventory: System: improve description

Remove the word enclosure from the description because an enclosure is
not necessarily a system (enclosure is likely a chassis).

Provide a few additional words of guidance, and elaborate on the purpose
of System.

Change-Id: I087866bbef4603472bafd2b45d63833eab79d174
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/xyz/openbmc_project/Inventory/Item/System.interface.yaml b/xyz/openbmc_project/Inventory/Item/System.interface.yaml
index 4df3dad..4be90b7 100644
--- a/xyz/openbmc_project/Inventory/Item/System.interface.yaml
+++ b/xyz/openbmc_project/Inventory/Item/System.interface.yaml
@@ -1,2 +1,5 @@
 description: >
-    Implement to provide system/enclosure attributes.
+    xyz.openbmc_project.Inventory.Item.System should be implemented on objects
+    that represent a computer system.  Implementing System does not provide any
+    additional API but instead adds type context to objects implementing other
+    inventory related DBus interfaces.