Document the scheduler

docs/reference/configuration.md

@@ -19,6 +19,7 @@ Anything already exported wins over the file.
 | `FLOWS_DIR` | `$DATA_DIR/flows` | the git repository holding flows |
 | `SECRETS_FILE` | `$DATA_DIR/secrets.enc` | encrypted credentials, deliberately outside the repo |
 | `ALERTS_FILE` | `$DATA_DIR/alerts.json` | channels and rules |
+| `PROVISIONERS_FILE` | `$DATA_DIR/provisioners.json` | clusters a machine can be started from; absent means none |
 | `PANELS_FILE` | `$DATA_DIR/panels.json` | wall-panel pairings |
 | `OAUTH_PRIVATE_KEY_FILE` | `$DATA_DIR/oauth-key.pem` | signs agent and worker tokens |
 | `CLOUD_CONFIG_FILE` | `$DATA_DIR/cloud.json` | the portal enrolment, if any |
@@ -141,8 +142,14 @@ A node that declares nothing is not accounted against `FLOW_CPUS`; it runs on
 the shared pool and is given `FLOW_CPUS / FLOW_MAX_WORKERS` as a thread cap, so
 several at once cannot each size themselves to the whole machine. Setting
 `OMP_NUM_THREADS` (or any of its siblings) on the engine yourself overrides
-that default. What is free, and which nodes are queued for it, is
-`GET /api/v1/workers/resources`. See
+that default.
+
+These two are this machine's figures. An attached worker reports its own when
+it dials in, and a node goes to whichever machine can grant what it asked for —
+so a GPU on a worker needs no `FLOW_GPUS` here. What every machine has free, and
+which nodes are queued, is `GET /api/v1/workers/resources` and the Workers
+screen. Named sizes live in the database and are `GET /api/v1/flavors`;
+`PROVISIONERS_FILE` is where machines can be started from. See
 [declaring resources](../getting-started/data-science.md#declaring-what-a-node-needs).
 
 ## Agents