docs: note session-rotation collapse in v4 upgrade guide

Author: barjin

docs/upgrading/upgrading_v4.md

@@ -198,6 +198,10 @@ await crawler.run([{ url: 'https://example.com', sessionId: 'basic' }]);
 
 More complex routing (more tiers, weighted draws, sticky assignment, cooldowns) can be expressed with additional named sessions and custom `errorHandler` logic.
 
+## `maxSessionRotations` and `request.sessionRotationCount` are removed
+
+Session errors no longer have their own retry budget. The `maxSessionRotations` crawler option, the `Request.sessionRotationCount` property, and the special-case retry logic for `SessionError` are all gone. A `SessionError` now retires the session and counts toward `maxRequestRetries` like any other failure, so configure a single retry limit via `maxRequestRetries` (default `3`). `SessionError` also no longer extends `RetryRequestError` - if you were catching `RetryRequestError` to detect a session-triggered retry, branch on `SessionError` directly instead.
+
 ## Remove `experimentalContainers` option
 
 This experimental option relied on an outdated manifest version for browser extensions, it is not possible to achieve this with the currently supported versions.