Docs: warn about the half-filled bucket, and say which node schedules

Say on the querying-chart page that a flow doing its own binning has to drop
the newest bucket, as the observability endpoints already do, and make plain
that `inject` is the scheduler while `trigger` is a debounce and hold.

docs/interface/dashboards.md

@@ -92,6 +92,15 @@ it, so swapping the store is a change to one flow and nothing else. The answer
 also states what it was computed for, so an answer to a different question is
 ignored rather than two charts overwriting each other's picture.
 
+!!! note "Drop the bucket that is still filling"
+
+    The request carries a window and an interval, and the binning is the flow's
+    own work — so the newest bucket only ever holds the part of an interval
+    that has elapsed. Drawn, it reads as a fall that never happened. The
+    engine's own [Activity charts](index.md#activity) end on the last closed
+    bin for that reason; a flow answering a chart has to drop or hold back its
+    newest bucket the same way.
+
 ## Media tiles
 
 A media widget draws what its message points at: a picture, a clip with