docs(rack): note render_power precondition (validate_power first)

scripts/gen_rack.py

@@ -298,7 +298,12 @@ def _node_id(name: str) -> str:
 
 
 def render_power(rack: str, items: list[dict]) -> str:
-    """Return a mermaid power-distribution flowchart, or '' if no feeds."""
+    """Return a mermaid power-distribution flowchart, or '' if no feeds.
+
+    Assumes `validate_power(items)` has already passed: every referenced PDU
+    resolves to a kind:pdu item with a positive-int `outlets`. `generate`
+    guarantees this by validating before any render call.
+    """
     powered = [fm for fm in items if fm.get("power")]
     if not powered:
         return ""