docs: add note on pipeline number priority vs. lock arrival order in serial groups

The existing documentation stated the priority rule but did not address
a common source of confusion in dynamic config setups: when setup workflow
duration differs between pipelines, lock arrival order and pipeline number
order can diverge. This note makes the distinction explicit and warns users
not to rely on merge order as a guarantee.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: hidetaka-cci

docs/guides/modules/orchestrate/pages/controlling-serial-execution-across-your-organization.adoc

@@ -49,6 +49,13 @@ CircleCI uses a distributed locking system to control execution order. When a jo
 
 CircleCI uses pipeline numbers to determine execution priority. Jobs from newer pipelines (higher pipeline numbers) take precedence over jobs from older pipelines within the same project. If a job from an older pipeline tries to run while a newer pipeline's job is waiting or running, the older job is automatically skipped.
 
+[NOTE]
+====
+Serial group priority is based on pipeline number, not on when a job reaches the lock. When a job attempts to acquire a serial group lock, it is skipped if a job from a higher-numbered pipeline in the same serial group is already queued or running.
+
+With dynamic configuration, setup workflow duration varies between pipelines, so the order in which continuation jobs reach the lock is not guaranteed to match pipeline number (that is, merge) order. A job from an older pipeline may therefore be skipped rather than queued. Do not rely on merge order to guarantee that every pipeline's serially grouped job runs.
+====
+
 === Job state transitions
 
 Jobs in a serial group move through several states: