bindings docs: mutations_before_rebuild is a floor, and the 26.10.1 workaround that lowers it does nothing #549

The published bindings docs (docs.humem.ai/arcadedb/latest) describe mutations_before_rebuild as an "override" of the rebuild threshold, and the vectors guide suggests "a lower mutations_before_rebuild" as a 26.10.1 workaround for ArcadeData#7260. The engine treats it as a floor: the threshold is max(mutationsBeforeRebuild, min(graph size x rebuildGraphRatio, maxPendingMutations)) (the same on 26.10.1, 26.10.2 and main), so lowering it changes nothing past a few thousand vectors. The knob that lowers the threshold is arcadedb.vectorIndex.rebuildGraphRatio, a database-scope setting set with ALTER DATABASE (checked on main c3af6d9028: 1,000 to 250 at 5,000 vectors). Fixed the same way in ArcadeData/arcadedb-python#111 and #113.