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/reference/node-types.md

@@ -126,7 +126,7 @@ thing configured elsewhere.
 
 ### Inject
 
-**`inject`** — emit a value on request, on a timer, or when the flow starts.
+**`inject`** — emit a value on request, on a schedule, or when the flow starts.
 
 | Setting | Default | Notes |
 |---|---|---|
@@ -137,8 +137,9 @@ thing configured elsewhere.
 | `at_start` | `false` | emit once when the flow starts |
 | `start_delay` | `1.0` | how long to wait before that first emission |
 
-The most-placed trigger in a real installation — mostly as a button someone
-presses.
+The scheduler: a `cron` expression here is what makes a flow run by the clock.
+It is also the most-placed node in a real installation — mostly as a button
+someone presses.
 
 ### Delay & schedule
 
@@ -169,7 +170,9 @@ needs a busy cascade slot waits for one.
 
 ### Trigger
 
-**`trigger`** — send one value now and another once things go quiet.
+**`trigger`** — send one value now and another once things go quiet. Despite
+the name, a debounce and hold rather than a scheduler: everything it sends
+starts from a value arriving. For a cron tick, see [`inject`](#inject).
 
 | Setting | Default | Notes |
 |---|---|---|