The help doc was rewritten and renamed "About Smart Bidding learning". It now separates the visible Learning status from the time bidding needs to stabilize.
Learning is triggered by:
a new or reactivated bid strategy
campaigns, ad groups or keywords added to or removed from the strategy
a significant change in the conversion goals you optimize to
Changes to budgets, targets, creatives or targeting settings (location, audience, language) never trigger Learning and never remove historical data.
Watch out: Google advises against several target changes within one conversion cycle, unless there is a clear business need.
The doc also lists best practices: one consistent primary conversion goal, enough daily budget, and consolidated campaigns to pool conversion data.
Spotted by Barry Schwartz, via Search Engine Roundtable.
