[DOCS] Adds further info to foreceast section in AD at scale. (#1356)

docs/en/stack/ml/anomaly-detection/anomaly-detection-scale.asciidoc

@@ -271,5 +271,7 @@ The memory usage of a forecast is restricted to 20 MB by default. From 7.9, you
 can extend this limit by setting `max_model_memory` to a higher value. The 
 maximum value is 40% of the memory limit of the {anomaly-job} or 500 MB. If the 
 forecast needs more memory than the provided value, it spools to disk. Forecasts 
-that would take more than 500 MB to run won’t start because this is the maximum 
-limit of disk space that a forecast is allowed to use.
+that spool to disk generally run slower. If you need to speed up forecasts, 
+increase the available memory for the forecast. Forecasts that would take more 
+than 500 MB to run won’t start because this is the maximum limit of disk space 
+that a forecast is allowed to use.