docs(historian): #691 bound walk-back cost after the #689 walk (rebase onto 88141d6e)

docs/Historian.md

@@ -794,8 +794,9 @@ switch `ServerHistorian:HonorReturnBounds = false` restores the pre-#691 behavio
   end bound ends the **last**: the pending end bound rides the continuation point, and when a page is exactly full the end bound is
   the whole next page. Paging a bounded read at any page size concatenates to the unpaged answer.
 - **How it is served — one authority, `HistoryBounds` in the OPC UA server.** "At or before T" is a newest-first read of `(0, T]`
-  capped at one — native on SqlServer/QuestDB, the #686 walk-back of bounded forward reads on the Gateway (so a tag with no sample
-  before StartTime walks back to 1970: the same ~14–16 small reads as a deep reverse page-through). "At or after T" is a forward read
+  capped at one — native on SqlServer/QuestDB, the #686 walk-back of bounded forward reads on the Gateway (a tag with no sample
+  before StartTime is proven empty down to 1970 by the #689 walk — after two empty segments it asks once for the whole remainder —
+  so a missing start bound costs a few small reads, not one per ×4 step). "At or after T" is a forward read
   of `[T, MaxValue)` capped at one — on the **Gateway** it ends at **now + 1 day** instead (the walk-back's horizon; the
   HistorianGateway only checks `start ≤ end`, so a year-9999 end would reach AVEVA), and an end bound beyond that horizon is
   `Bad_BoundNotFound` without a read. SqlServer/QuestDB keep `MaxValue` (they bind it safely and may hold samples further ahead).