doc: describe the permit bound as the fraction it now is

The design document described the permit fraction in nano-fractions of the
worker count. The parameter is a plain `f64` fraction, so the description
matches what an operator sets.

Author: antiguru

doc/developer/design/20260825_peek_execution.md

@@ -406,11 +406,11 @@ Four parameters:
   whose rows are bound for the stash suspends once its accumulation crosses
   `peek_response_stash_threshold_bytes`, by far the smaller trigger at that
   parameter's default, so such a walk yields per batch.
-* **Permit fraction**, default one permit per timely worker, expressed in
-  nano-fractions of the worker count the way Kubernetes expresses a CPU share.
-  How many promoted scans may run at once. It bounds running scans and nothing
-  else, and the queue that forms behind it is a signal rather than a second
-  bound.
+* **Permit fraction**, default `1.0`, a fraction of the timely workers a compute
+  runtime runs. How many promoted scans may run at once, floored at one scan.
+  Expressed as a fraction so the bound scales with the replica rather than being
+  retuned per size. It bounds running scans and nothing else, and the queue that
+  forms behind it is a signal rather than a second bound.
 
 A peek passed over for want of budget is served before the peeks that were served
 ahead of it, which takes two pieces beyond the counters. The sweep records the