Clarify Upstream/Downstream naming convention in tlbmc documentation

tlBMC uses a different naming convention for describing upstream and downstream ports than traditional Entity Manager. Add a clarifying note to onboarding documentation to clarify this difference.

#tlbmc

PiperOrigin-RevId: 778255318
Change-Id: I610a18e34ebf92d79a4402a76383ae59b9190975

tlbmc/g3doc/general/onboarding_npis.md

diff --git a/tlbmc/g3doc/general/onboarding_npis.md b/tlbmc/g3doc/general/onboarding_npis.md
index 6807467..4c73f76 100644
--- a/tlbmc/g3doc/general/onboarding_npis.md
+++ b/tlbmc/g3doc/general/onboarding_npis.md
@@ -156,6 +156,13 @@
 `CableUpstreamConnection`. This is enforced by tlBMC and the store will fail to
 create if this condition is not true.
 
+Note: In contrast to Entity Manager, "Upstream" and "Downstream" in tlBMC refers
+to the relationship of a component that is upstream/downstream of the component
+where the port exists. For instance, a root chassis has no `UpstreamConnection`,
+but can have many `PortDownstream` since all other components must be downstream
+in relation to the root chassis. In traditional Entity Manager,
+upstream/downstream refers to the role of the component directly.
+
 An example of a `PortDownstream` port:
 
 ```